If you want a lift a human to do something, you also need some better steering and safe landing, and those require more reliable engineering besides just the basic lift possibility.
Kind of supports my question - if tethered kites, why not untethered ones (i.e. aircraft)? And man-lifting kites have, of course been a thing for ages.
Reliable steering requires aerodynamics requires quite a lot of mathematical development, plus considerable precision engineering. And it's extremely dangerous, a lot of people did die trying to make steerable kites.
The Wrights did extensive experimentation, on both models an full-sized prototypes. They built the first wind tunnel and developed calculations for lift and propeller thrust.
They also practiced fairly rapid design iteration, trying out ideas and adapting to actual experience. What they arrived at worked, though it was far from ideal and doesn't much resemble modern aircraft (beyond the notion of wings and the rough principles of control surfaces). Once other designers / engineers entered the field, and with more reliable engines, further iteration advanced rapidly.
Military aircraft played a significant role in WWI, only a decade after the Wright's first powered flight.
> He identified the four forces which act on a heavier-than-air flying vehicle: weight, lift, drag and thrust. [...] He also designed the first glider reliably reported to carry a human aloft. He correctly predicted that sustained flight would not occur until a lightweight engine was developed to provide adequate thrust and lift. The Wright brothers acknowledged his importance to the development of aviation.
I think by the time the Wright brothers came around, the general theory for powered flight was in place. The Wright Flyer was precision engineered, compared to most 'kites' that came before it. It's not every day you see an internal combustion engine on a kite.
> It's not every day you see an internal combustion engine on a kite.
Yup. The engine was more important than the understanding of the principles of propulsion too: even if the four forces had been identified by Aristotle, that wouldn't have been much use in achieving sustained powered flight to civilisations whose closest approximation of a propellor powered by a turbine engine was a waterwheel.
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> Then, why not make it bigger?
To what end? They were lifting bombs on kites as early as the 7th century, as well as humans as novelties (and punishments - https://en.wikipedia.org/wiki/Yuan_Huangtou).
If you want a lift a human to do something, you also need some better steering and safe landing, and those require more reliable engineering besides just the basic lift possibility.