Cultures where people still sit or squat - floor or toilet - tend to be tropical or equatorial.
Temperate or arctic people like to be away from cold, damp positions; for understandable reasons.
I'm not quite sure that's the whole story, Jude. Mongols, Inuit, Koreans, and Japanese all have homelands with cold winters, and are all traditionally floor-sitters. Bedouin Arabs inhabit places that are hot every day and cold every night, and they don't break out the chairs when night falls. One of the major reasons carpets and floor mats of all sorts were invented was to enable people to sit and lay on cold ground. Mexico and Brazil are tropical countries where the chair is quite popular, though I'm not sure if this was always the case traditionally.
I'm actually kind of curious to see what sort of practical consideration held the most sway in determining whether groups of people adopted the custom of sitting with hips and knees high above the ground, or close to it. My brother in law is an architect, and learned in architecture school that every decorative or seemingly vain detail of architecture and design originated in something practical. For example, wainscoting evolved from the wood planks used to insulate the thick, cold stone walls of castles, and cupolas were for ventilation. I wonder if sitting a few feet off the floor, with knees free to dangle and feet able to rest on the floor in the same position as when standing, was at one time necessitated by the way people's houses, clothes, or even bodies were in certain places.
If the floor is a place for people to sit and/or lay, taking off your shoes indoors and maintaining good foot hygiene takes on a salient importance that it needn't in places where the floor is not directly sat upon. I can also see why kicking people or even nudging or pointing with the feet in most such cultures is appallingly rude.
Thujone, sitting on the floor can be perfectly ergonomic, not putting any excess strain on any joints, with nothing more than a firm cushion under your keister that goes no farther forward than your sitz bones (ischial tuberosities). There are probably a number of other fairly simple techniques and tools that make sitting very low to the ground as tolerable as sitting in the average chair. But ultimately I agree with you -- prolonged sitting isn't healthy, period. It would be interesting to see if anyone has studied the incidence of blood clots in the legs of peoples who habitually sit on the floor or very close to it, versus ones who typically don't.
What would the car look like, had it been invented by a nation of people who always sat on the floor? Would the bicycle, likewise, have evolved very differently?