In FPS games the gameplay is a lot more ephemeral. You're racking up point after point, and your teammates are all interchangeable. In DOTA, there's only one real point. Your teammates are people you're stuck with for the entire duration of the match. And you can see their every move, no matter how far they are from you. And the worst player? The scrub? He's not just ineffective, he's actually feeding power into the opposition.