What motivates them? Having a platform, probably. I see it on a lot of the game-specific forums I frequent too - you'll get a certain band of kids (at least, I hope they're kids) endlessly bashing the latest installment of a particular series and how anyone who happens to like it is a shill and not a true fan because any true fan would know that it's RUINED FOREVER (the main forum which covers a popular sandbox crime game franchise is particularly guilty of this
). Maybe they just get off on the backlash they generate. Experience has taught me not to get involved.
What I will say is that the aforementioned forum does have a lot of those people whining about their "harmless" hacks/exploits being patched out and how they're being punished for "just having fun". Yeah, right.