As a guy in a frat I can give you why my house specifically has that rule.

Essentially, the reason is that, as you’ve heard, guys are a liability. Girls present their own liability but in a different sense. If you let in a guy, you’re not really worried about them breaking shit or stealing shit and whatnot. You would idiot-proof your house beforehand so that isn’t an issue. Instead, you mainly run the risk the this rando you’ve let in does creepy shit. In my house, we’d have two sets of open parties— welcome week and rush week. These were always super stressful. The number of guys who you’d have to kick out because girls were complaining or you saw them do something off was uncomfortably high. Bars don’t have this problem because if a girl has a shitty experience at a bar, you can’t really point the finger at the ownership. However, the fraternity brothers are tied to the house, so if something bad happens to a girl (and it’s way more common at frats unfortunately) then you carry that reputation because she has no idea if he’s in the frat or not.

I know there’s a LOT of seriously-not-wholesome shit that happens at frats, and as such frats take lots of very legitimate criticism, but at least in my house specifically, this rule was just genuinely about wanting to keep girls safe. We know the guys we have aren’t creepy, and feel more comfortable not rolling the dice. On the flip side, there’s really no concern with girls. You’re worried about them stealing shit and breaking stuff but you just lock most doors and you have sober monitors watch out just in case. You’re never worried about a girl assaulting a guy, cause as sad as it is to say, that stuff really only goes one way in these contexts.

I can’t speak for other houses. I’m sure as a general practice, the prestige and exclusivity plays a huge role too. There’s some smaller things— for instance, our finances are always kinda tight cause these parties are expensive, and I’m damn sure not paying for some random dudes cocktail with my dues so he can hit on my girlfriend. But, the main reason is safety, regardless of what other people in the thread say.