I'm with you op. It's used to demean and degrade and diminsh.
Even those who do it 'through habit' or without thinking, that's where it comes from, and it's no excuse.
Context is key. I might have a night out 'with the girls', same as DH might go out 'with the boys' (having typed that, I don't think either of us say that but...). But I am a grown woman, not under 18.
I've asked a couple of people over the years why they refer to men as men and women as girls.
One (a man) said 'well 'woman' sounds rude doesn't it'. Unpicking that, 'woman' has, in some cases, become derogatory, used as an insult (it's not hard to think of the connotations where woman has become part of an insult, linked with mosogyny). Whereas it's just a plain description of who we are.
Another (a woman) said "woman sounds like you're old, don't you want to still be a girl with all that suggests?". No - I am not embarassed about being a grown woman.
At work I asked why people referred to 'the girls on reception' and not 'the boys in IT' (who, incidentally, were all much younger than the 'girls' on reception. Noone could explain that one.
Whether or not you're a royalist, they choose their language carefully and always with respect. When the Queen died, none of her family refered to her as a 'lovely girl' and her children spoke about her being a 'strong woman', 'loyal woman', 'determined woman'.
A lot of people will say they don't mind it, but it's casual sexism.