People could also sit there and re-type the whole thing if they really really really wanted to steal it. I’m sure there’s other high tech methods which are easier, but that option is always available no matter what Wattpad does to protect it from being stolen.
As for why… well I’m part of the copyright team and my job is to go and find published books that have been posted to Wattpad and report them. Unfortunately only the original copyright holders can report the non-published books. And I’ve seen many people saying things like “I’m posting this here so that those who can’t afford it can read it” so they genuinely think they’re doing something nice without knowing (or maybe caring) of the legal ramifications of what they’re doing. I’ve had books removed because all the poster did was change character names and call it a fan fiction and I’ve seen the poster complain about Wattpad just trying to ruin their fun. Those people usually don’t realize how much legal trouble they could be in if the publisher of the book ever found the book instead of Wattpad just deleting it. It’s one of those if it didn’t happen to someone else, that means it won’t happen to me kind of deals.
Others steal because they don’t think they’ll get caught and they like the attention they’re getting being a “writer.” I think for those it’s the thrill of it that gets them to keep doing it.