In grade 7, I was minding my own business in some sweats, and my best friend pantsed me. Have never worn sweats outside of warmups for the gym, ever again.
Also, it's an easy way to make a basic outfit interesting. And you don't have to worry about them getting dirty, overused, whatever. They don't wear out, and generally, they look better the older they get.
Honestly, what's to not like?