I'd hypothesize that as the provider men shower in the morning for work. Although America no longer is reliant on the male sole provider social order, the lingering effects remain.

OMG @1 what century do you live in? Before the pandemic there were more women than men in the work force.

https://www.cnbc.com/2020/01/14/why-more-women-are-working-in-the-us-than-men.html

The pandemic has taken nearly one million women out of the workforce, however women have been working since the beginning of time (it was only wealthy white women who were SAHMs). Black women (and all other non-white women) and countless white women who were poor worked their entire lives. If my grandmother (who worked a factory line her entire life and started working when she was a young teenager) were still alive, I'd ask her if she showered in the morning or night and that would not make a scientific observations, either.

How about it's just a personal preference?

There are an infinite number of reasons any given person, regardless of their sex, showers when they shower. One IA's observance (which they call sexist, which makes no sense, as when one showers has nothing to do with sexism) does not a scientific observation make.

But if you want or need some science here it is:
Most of the science, it seems, falls on the side of the shower at night camp for two main reasons: Cleaning your skin and helping you fall asleep faster.
https://www.wideopeneats.com/shower-at-night-or-morning/

It turns out, the evening washers. Of course, if you want to shower in the morning there’s nothing to stop you, but you should absolutely shower before bed.
https://www.independent.co.uk/life-style/health-and-families/when-best-time-shower-experts-reveal-morning-evening-sleep-better-a7724151.html

An actual sexist interpretation (if more women than men shower at night) of the science would be - more women than men believe in science and therefore are smarter than men and that's why they shower at night.

Oh and one more thing: (which is in fact sexist and still exists to this day) since most women with children (even those who are married and work full time, not just single mothers who work) do all of the child care needs in the morning - waking the kids up, getting them dressed, making them breakfast, taking them to school, etc.that might be another reason why (if it's true) more women shower at night than men who have the luxury of showering in the morning.

Well done Chrstina Rae - Exactly the response I expected.

OMF.
I am a guy & I sweat at night, like enough to turn the pillow over & then switch to another pillow (ik: privelege) so I used to shower in the morning to be presentable throughout the day.
Bride (fem.) would barely awaken fresh as a freak'n daisy & not need to shower.

One male's opinion: it was just ingrained as part of the morning ritual growing up, part of the waking up process along w breakfast. I will also second what another poster said that there is some night sweat that it helps wash off and helps you feel like you are starting your day fresh. The women I have asked have said it is either because of their hair or they like to go to bed clean.