Because he is a kid, hes naive, and is still learning about the HUGE amount of stuff going on. He's not quite the brightest person in the series...especially in the second game.
I call Donald stupid because he CONSTANTLY runs into enemies and DIES instantly. And Goofy has too long of an HP bar at the end of the games to die as instantly.
I wouldn't say that people portray him as dumb, I would say it's more like he's just a young and naive kid. He's still growing up, he's kinda immature and he is always so cheerful, which could make him seem like not the brightest dude in KH, but I wouldn't say that he's dumb.
I'm not gonna be portraying him dumbin my Fanfic. Donald and Goofy though......total stupidity without my help.XD as for why other people do portray him as dumb...its not dumb so much as naive, and trying to have some fun. he kid.... we must remember this...(of course in my fan fic he's an adult so.....really its makes no sense for me to even say I won't portray him as stupid in my fan fic)
