[Jstylezzz]

My guess is that a big part (if not the majority) of members are like, very underage. I'm not saying all 13-year olds are incapable of showing normal behavior (I have seen enough cases in which even 20-year olds were more immature and rude than their underage counterparts lol), but I do believe this is one of the reasons. For some of them the SA-MP forums might be the first forum they have ever been on. Combine this with your reason that GTA:SA is easy to crack, combine it with stubbornness and the will to start a server no matter what because that's ultimate power, and you have a wonderful blend of the thing we see on a daily basis here on the SA-MP forums.
Although I do also believe that age is not always a leading reason in this case, this is what my best guess is.