The answer to the OP's question is obvious: because they have done it all their lives just as their ancestors have done it in this part of the world since time out of mind.
As kids growing up in a semi-rural area we played on the floor a lot. Even when older... board games and such.
The floor, if it is made to be inviting with oriental rugs or even a bamboo mat is a good place to do many things.
Why more people in the West don't use it more often is probably due to the Faranglander's barbaric habit of wearing their shoes inside the house.