Why do mods do this?

[callforjudgement]

It's fairly critical to ensure that only the sort of player who enjoys the sort of game you're planning to run will sign up, or else you'll have a lot of upset players and replace-outs. Thus the information given in signups. (Note that "surprise me" is a sort of setup too, and some that many players enjoy, but you have to make it clear that players can't rely on any particular expectations.)

It's normally fairly easy to balance around any information you might have to give.