Depends on the person and many do it just because the majority of people nowadays dress down. Its basically just to blend in with the average person. I mean they are real people just like you and I. If they like jeans and t-shirts during the day, then more power to them. Some still dress in dress clothes all the time. One guy that comes to mind was Vinny Gorgeous but he's in prison now.

If they go out for dinner on Friday night and hit the best lounges in New York, they will dress up. You will see them in their best clothes looking like they stepped out of a GQ magazine.