One size fits all cannot be average, it has to be tiny. You can fold a tall person into a small bike, but you can't stretch a short person onto a large bike.

There seemed to be a series of Chinese companies that offered particularly small bikes in Paris. Ofo and Mobike were (are?) just tiny. Maybe they came from a market where the average person was significantly smaller? The bikes were pretty much unusable on a shallow incline if you were of even average height. For comparison, my 9 year old child has a bike that has a larger frame than those ones.

It seems safer for a big person to ride a too-small bike than a small person to ride a too-big bike.