Why does a lot of americans use gym clothes and pajamas to any place that doesn’t have a dress code?

I know that not every american does this, but you guys do it a lot more than other nationalities.

Edit: some answers really do make a lot of sense (health, remote work, culture, etc), but I'm getting the impression that americans spend all their free time outside the house at grocery shops, lol. Also ppl seems to assume that anything dressy is automatically unconfortable?