Firstly, this is easier said than done.

User reports are a dangerous step to take, because once they prove they do it, any company can just review bot their competition claiming it's fake.

They could technically police their own ad networks, but most of these networks are not Apple's so they can't. They'd have to just hire people to go play games to get ads to click on to then take down games.

And then what's the point? Apple is just money chasing like every other company, and most of the huge game companies do this. They'd be shooting themselves in the foot and hurting their own revenue. As much as they like to tout that they protect users, that's something they like to say because it serves them. At the end of the day, their own best interests are far more important to them.