TL;DR: That's largely not true and just a dumb observation some guys make because they're (sometimes justifiably) annoyed. Women put effort into their outfits for the same reasons men do.

Long version: Really? Because I've seen and heard so many women express embarrassment to other women for how much effort they were putting into an outfit and makeup only to end the night without meeting a guy, or because they came with a guy and are starting to feel like all of the effort makes her look desperate for his attention.

Maybe earlier in life like in high school some girls only put effort into their outfit to impress or compete with their peers, but as adults women put effort into their outfits to go out either because 1. they like the way they look/it makes them feel more confident (same reason for guys), 2. they know the friends they're going out with are also putting effort into their outfit and they want to fit in with their group/don't want to be the only one not trying/don't want to be the only one not getting attention (all the same reason for guys), or 3. because they're trying to look hot because they're going to place that might attract people they're attracted to (Again, same reason for guys).

The whole "women dress up for women" is USUALLY an inane observation from some guys because of how annoyed they get at witnessing or being involved in the effort their partner puts into her looks. You're trying to not miss the beginning of a movie and their effort into their outfit is making you late, they're asking you about what shade of lipstick they should wear when you don't care, they're switching between outfits to ask what you like better and you'd rather be doing anything else, etc. Eventually all of those little things build up and some guys come to the conclusion: I don't know why she's going through all of this, it sometimes puts her in a bad mood because nothing looks right to her, I'm realizing I'm constantly saying I don't care or I don't know, I'm realizing I'd rather she wear no lipstick at all (for example) if it meant we could be on time, I don't think she's trying to leave me for anyone else..... Oh I know, she must be doing it so she looks good to other women! Especially if the guy has himself stop putting in effort into how they look when they go out, opting for comfort since they already have a mate.

Those same guys are usually the first ones to bitch and moan when their partner goes several months without dressing up, especially if it's been awhile and they see one of their guy friends with someone who is dolled up and they remember their girlfriend/wife used to dress up nice and hasn't in awhile.