What amazes me about heavier than air flight is that it was invented all over the planet at virtually the same time by all kinds of unrelated people who had no connection or communication with or little knowledge of one another. Sure here in the US we ethnocentrically recite the Kitty Hawk event of the Wright Brothers as if it appeared in a vacuum out of nowhere. But in reality, heavier than air flight was invented simultaneously (by the standards of the time) at multiple places all over the world from New Zealand (March 1903) [1] to the USA (December 1903) [2]. It's as if human consciousness just exploded with a new-found ability to fly all over the earth all at once.
Closer examination shows that like most human inventions, these breakthrough moments are the final incremental accumulation of ideas that were a long-time coming. In the case of flight, there were centuries of lighter than air flying methods based on buoyancy and displacement [3]. Then gradually an understanding of fixed wing aerodynamics evolved (e.g. the impressive work of people like George Cayley [4] and Otto Lilienthal [5]). It was the parallel development of combustion engines that made the Kitty Hawk and Waitohi moments eventually possible (which I think might really the answer to your question "why heavier-than-air flight was not invented earlier?"). While the invention of the steam engine gave rise to the entire industrial revolution, piston steam engines of the time were too heavy to power flight. The internal combustion engine was finally applied to the automobile by Karl Benz in 1885 [6] and within the relatively short span of 18 years had evolved to the point where powering an aerodynamic surface was feasible. Just 66 years later man walked on the moon.
It’s pretty common that “breakthrough inventions” are arrived-at pretty much simultaneously by apparently disparate sources at approximately the same moment in time.
Consider the telephone, which is attributed to Alexander Graham Bell in most of the world and to Antonio Meucci in my native Italy. Or radio, which is broadly attributed to Marconi or Tesla. In hindsight it seems like one person triumphed upon others, but really if you look at it from their point of view they work with urgency and secrecy because they perceive themselves to be in neck-to-neck competition with their cohorts. They perceive their technological environ very differently from how we do ex post facto.
I think that what you say is probably true, but things like thermal-riding or slope-soaring gliders would have been very useful (for the military, if nothing else) before the invention of the IC engine.
As discussed further down, kites were indeed seen as useful for the military in the late 19th century. Wikipedia says[0]
> In the early 1890s, Captain B.F.S Baden-Powell ... developed his "Levitor" kite, a hexagonal-shaped kite intended to be used by the army in order to lift a man for aerial observation or for lifting large loads such as a wireless antenna.
A glider may not have been suitable for carrying an antenna, but aerial observation via glider is an idea that must have occurred to people in the army, even before the invention of the IC engine.
Big questions is getting those to air, what you actually do with them and then how to solve communicating from them. Just think of how very early WW1 planes were actually used.
> who had no connection or communication with or little knowledge of one another
There was a large enthusiastic group of people working on heavier-than-air flight, with with meetings and newspaper publications. The Spectator has an article titled "Flying Motor-Cars" at https://archive.org/details/sim_spectator-uk_1901-08-31_87_3... which comments:
] The mechanical skill of the world, which is very great, greater perhaps than its originality in scientific investigation, is directing itself for the moment to two definite ends, — the construction of an efficient submarine boat, and the invention of a machine that can travel with at least two persons on board through the air.
That the Wright Brothers didn't know Pearse is besides the point - both drew from shared materials, and a lot of people were trying. Here's a couple of reports from the New York Times:
"TO FLY FROM PIKE'S PEAK.; W.F. Felts Tries His New Aeroplane at Different Altitudes. [Aug. 4, 1897] (followed soon by SNOWSTORM ON PIKE'S PEAK.; W. B. Felts Did Not Attempt His Aeroplane Flight Yesterday.)"
Or "EXPECTS TO BE ABLE TO FLY.; Prof. Bell Believes He Has Mastered the Two Great Difficulties of Aerial Navigation.". That's Alexander Graham Bell.
Your [4] even mentions 'The Wright brothers acknowledged [Cayley's] importance to the development of aviation'.
> here in the US we ethnocentrically recite the Kitty Hawk event of the Wright Brothers as if it appeared in a vacuum out of nowhere
] In 1896, when he was twenty-five years old, Orville was very sick with typhoid fever and almost died. Wilbur and Katharine cared for him. Wilbur read while sitting with Orville. He read about Otto Lilienthal, who was trying to fly. Lilienthal built gliders and had flown farther than anyone else in the world. But Otto Lilienthal had a gliding accident and died. The Wright brothers were saddened by this news because they admired Mr. Lilienthal and his attempts to fly.
] ... In England, France, the United States, and other countries, people were trying to unlock the mystery of flight.
] ... Wilbur learned all he could about flying. He took every book about it out of the Dayton library. Samuel Langley the head of the Smithsonian Institution, was trying to learn how to fly Wilbur decided to write the Smithsonian. A man there sent Wilbur information.
] The famous engineer Octave Chanute was also experimenting with gliders. Wilbur wrote him, too. Mr. Chanute quickly became a friend of the Wright brothers.
It's pretty common for things like that to happen. There are many other cases. Calculus from Newton and Leipzig being another example. There is even a wikipedia page delving into the concept: https://en.wikipedia.org/wiki/Multiple_discovery
Basic hypothesis is that once enough precondition for an invention are there then the leap can be made.
I stared for a long time at your third sentence before I remembered that Leibniz is the correct name of the German mathematician you are referring to, and Leipzig is a German city (where, coincidentally, he was born).
Closer examination shows that like most human inventions, these breakthrough moments are the final incremental accumulation of ideas that were a long-time coming. In the case of flight, there were centuries of lighter than air flying methods based on buoyancy and displacement [3]. Then gradually an understanding of fixed wing aerodynamics evolved (e.g. the impressive work of people like George Cayley [4] and Otto Lilienthal [5]). It was the parallel development of combustion engines that made the Kitty Hawk and Waitohi moments eventually possible (which I think might really the answer to your question "why heavier-than-air flight was not invented earlier?"). While the invention of the steam engine gave rise to the entire industrial revolution, piston steam engines of the time were too heavy to power flight. The internal combustion engine was finally applied to the automobile by Karl Benz in 1885 [6] and within the relatively short span of 18 years had evolved to the point where powering an aerodynamic surface was feasible. Just 66 years later man walked on the moon.
[1] https://en.wikipedia.org/wiki/Richard_Pearse
[2] https://en.wikipedia.org/wiki/Wright_brothers
[3] https://en.wikipedia.org/wiki/Early_flying_machines
[4] https://en.wikipedia.org/wiki/George_Cayley
[5] https://en.wikipedia.org/wiki/Otto_Lilienthal
[6] https://en.wikipedia.org/wiki/Carl_Benz