Most of us look fairly clean at the beginning of the weekend, but by the end of the weekend there are usually some new patches (right Doug?) and lots of new or bigger sweat stains.
Seriously though, you're right most reinactors' uniforms do look too clean and too new. I wash mine when they get dirty and patch them when they wear through. They look new and clean when they are new but the longer I have them the more wore in they look.