i don't know, same reason you would get dressed up to sit around the house alone on a day off i guess.
I wear a uniform too. On the weekends I dress in jeans and a black metal shirt with an unreadable band name on the front. That's me.
I would hate that. I can do coveralls but not the office look. It's why I managed to avoid working in any offices. When I was in the military we wore slack clothing, called a work uniform that we could do anything in and only wore the suit bit for an occasion or a parade which I rarely ended up on. I was sneaky and managed to avoid most of those by volunteering for something else, like covering a shift or planning my holidays when I knew something could come up. I hated anything tight around my neck. The work uniform was almost as comfortable as jeans and a loose shirt and on some small stations we could wear combat boots so I wore mine till they were almost as comfortable as moccasin boots. I wore them for many years at other places I worked after I quit the military. The soles were finally so thin I could feel the ground through them then they got tossed once holes started to show up.
If i was just sitting around the house on my day ofg i wouldnt be wearing anything. except in the winter...but i would pick clothes by warmth in that case, looks would not factor in.
so you basically dress like an auto mechanic with grass stains 24/7 .. its all gravy.. on your tshirt..