Lanefan
Victoria Rules
In part because doing so would make it too easy to metagame before the characters have had time to figure things out in the fiction. Sometimes foes are tougher or easier than they look, never mind when you've got illusions involved.It’s untenable because just by taking part in the game and simple observation, the players will learn the rules. Or, many of them, at least.
Like the hidden AC. Forget that AC represents things that are mostly observable, so sharing it seems reasonable… after a few rolls in combat, it’s usually pretty obvious what a creature’s AC is. “Oh, I missed when I rolled a 14, but then Tom hit when he rolled a 15.” It doesn’t take a rules lawyer to figure that one out.
So why not just share it ahead of time?
For example, it'll usually take a round or two for characters to figure out roughly how difficult it is to hit their foes; during that round or two the players would highly likely take a different approach if they already knew the foes' AC, as in "This guy's only got AC 12? Hell, it's Power Attack all the way!" rather than taking a round or two to come to that conclusion. Or "He's only in leather and he moves like a brick yet he's got AC 18? I'd better buff up my to-hits rather than my damage", which doesn't allow space for the player/PC to buff the wrong thing due to deceiving appearances.