People who don't care about consistency in their results.
Lost 5 games? Won 1? Oh, you won 1 game when they finally swapped characters, you must be better than them, better quit before they beat you as that character, too!
It's just kind of ridiculous. I don't mind when people are naturally the type to swap and swap again, or quit after X amount of games, but when people take losing as the only factor as to if they should swap/quit or not, it's kinda pathetic. I've played against people who'll stick to a character, get a blatantly lucky win (*cough* Footstooling after whiffing d-air as captain Falcon twice. *cough*), then not swap only to get stomped and swap characters, just so they can get stomped again and quit.
Maybe if you just stuck to learning one character to be good as, you'd be better, people don't get that. -and yes you can argue "If you're so good you could beat me as any character! Stop only playing ___.", but then you have the flipside, which is "If you're so good, you'd be more confident of which character is your best, and you'd keep trying to beat me as them."... (Unless you're Nairo and go ham with Doc. Harr harr de harr harr.)