I'm sure they wash their clothes regularly. It's probably easier for them to pull on the same outfit every time instead of constantly searching for a new one. You don't see them going to the toilet either or sleep much, because that doesn't add anything to the plot.
Plus well with the war now going on, I think that changing clothes is the least of their worries now ^^;
And it would be a pain in the ass for Kishimoto-sensei to constantly draw new clothes.