Sometimes, people don't have the time or inclination to make an interesting costume. They might not have the finances, either. I know one guy who wore his cook's uniform to a costume party, for example. That was reasonable, especially since he wasn't earning a lot of money.
However, it's usually lame when people take these costumes and actually enter a contest. For example, I attended a costume event once wherein one guy wore his taekwondo outfit. Nothing wrong with that, but then he entered himself into the costume contest. Predictably, people did not applaud for him. No offense to the guy, but it's not hard to see why. There's nothing clever about that kind of costume; instead, it speaks of someone who just used whatever he had lying around and using it uncreatively.
BO-RING.