But yeah, in some natural environments, with some kinds of bikes, you can beat walking. The Burning Man festival is a great example, but it is also a terrible place to live.
That's why in most cases, without smooth roads, bikes are not practical.
I think it's fair to point out that you should be referencing road bikes then when you say bikes are not practical. Advancements in tubeless technology, suspension, MTB groupsets with dinner plate low gears, derailleur clutches, and hubs/rims designed to take a beating while supporting wide tires can definitely make a bike more efficient than walking on most terrain.
- Fat bikes are more efficient than snowshoeing or breaking trail on XC skis.
- CX bikes are more efficient than walking in mud.
- Fatter tire gravel bikes are more efficient than hiking through sand.
- A bike with a 51T cog and 28T ring will be more efficient than hiking up steep grades until balance at low speed becomes an issue.
https://worldbicyclerelief.org