> How is AI supposed to simulate a player, and why should it be able to determine what real people would find engaging?

Games have goals, and players are prone to 'optimising the fun out of games', by doing some save strategy over and over again to reach that goal, even if it's not fun. Think eg grinding in an RPG, instead of facing tough battles with strategy and wits and the risk of failure.

Even if AIs are terrible at determining what's engaging, you can probably at least use them to relatively quickly find ways that you accidentally opened that let players get in the way of their own fun.