all my Planets seem to get a stealth bonus of +100 for EACH cultural archives I control.
so is there any word on why AI HWs have such idiotically high stealth values?
The stealth seems to be ridiculously high because all the planets gain +100 stealth for each cultural archive in the whole map, regardless if they are visible or not. This includes native populations that have cultural archives as well as each of the AIs in the game. You also don't have to control that planet to get the boost (I think this was overlooked). The unknown factor that adds to the stealth is actually the bonus coming from all those archives. As you uncover the archived planets, you will notice that the total stealth doesn't change; the info box will list each of those boosts from those planets. Because of this, if you want to play a game where you can actually take over the AIs, you'll have to check the stealth value your homeworld has and make sure it's 775 or less (max detection possible is ravenous hull + sensors = 825; possible laenfa population: -50). You can change the native density if the stealth is too high.