Because fashion brands make money from women’s insecurities and desires (for love, for a 2.0 version of themselves, for a better life). So they have a huge monetary investment in women associating consumption/new clothes with feeling slim, confident, loved, brave etc. Feeling like the person you want to be.
So when women shop together and they find a good flattering piece, in the mirror they see themselves with new eyes, they feel good about themselves.
The same can happen to men but is less common, both because they tend to have invested less in developing a personal style and because they are less aggressively a target of advertisement. This has a long history, too, like how back in the fifties mental health was famously poor and women used “mommy’s little helpers” or Milltown or whatever, but luxury kitchen articles and fashion were branded as fulfilling and a source of satisfaction, joy, and pride. Bored housewives fantasizing about life being different are much easier to tempt into spending cash than men with less time and better professional and social lives.