OK I will go ahead and mention the elephant in the room - I think it's because men are assumed (not incorrectly in many cases) to be the ones paying for the purchases. Therefore if they get a bag they are willingly paying for it right then and there with their money whereas a woman is perhaps getting permission from the husband to spend the husband’s money.
I literally have been asked "is your husband ok with you spending like this" by SAs (H and other brands, and in different countries). I actually earn more than my husband and am the one paying for my luxury purchases (I have decided to always go shopping by myself too) but I do get why. It's still the default way of thinking that men are the ones providing the funds, because of how things used to be (I remember when my mom can't even *own* a card without going through her husband), and of course, in many cases still continue to be (gender pay inequality is a thing). And it definitely can be true that the husbands are picking up the tab (how many posts have we seen that mention "my husband is going to put me on ban island" on this forum?). I have also personally ran into someone at my store hiding her purchase (like a small leather good into her pocket) and joking (and I hope only just joking) with the SA to not have her husband angry at her with the H shopping.
It's like when husband and I go out to eat at a restaurant and the waiter defaults to giving the check to him instead of me. IMO it's the same assumptions at work. Is it sexiest? Yes that is literally what this is, but I don't think at the root it is (simply) an H thing or an SA (/waiter) thing, rather it’s reflecting on the still existing gender roles and inequalities in cultures and societies as a whole.
P.S. an aside - I am not saying that there is anything wrong with people spending their partner's (or parents') money or whatever. As long as the ones providing the money are happy, I don’t think it’s up to anyone else to judge. I wouldn’t say no if my husband wants to buy me H or anything - in fact he has done it once and it’s my most special piece that I will never get rid of