Our poor posture and use of the body in the West is partly a side-effect of our mental activities, in my opinion. Not merely due to sitting. Rather sitting and performing mental activities whilst partially tranquilised:

e.g. playing video games at a desk whilst sipping coffee/soda/alcohol.

I can't explain why this is but intuitively I think it's to do with our locus of attention being out of the body and into abstract ideas. In the developing world their locus is more 'in the body'.