Because there’s no good equivalent to “guys” for women, and women often feels too old/formal. If I’m talking about a group of 19 year olds, then they are women and men (and there’s no good word for NB adults, other than “adults,” that I can think of, either), but 19 year olds still feel younger than women and men. “Guy” is any age and denotes a peer or relaxed relationship, but “woman” and “man” don’t have those connotations. I would talk about the man who works at the bank and the guy who works at the coffee shop, as an indicator of familiarity, if that helps. If you speak a language with a formal you and an informal you, it feels like a similar distinction to me, though those are also all different.
“Guys” can refer to groups of women, and I definitely call my sisters guys, but if you talk about “a guy,” it isn’t gender neutral where I’m from.
“Lady” singular denotes age, but not formality, though the formality difference between “lady” and “ladies” is hard (I could absolutely see someone saying “some lady was an absolute asshole at the gas station today,” but “two ladies were absolute assholes at the gas station,” is weird).