[1] https://www.census.gov/library/stories/2022/03/what-is-the-g...
It was the norm for generations because women didn't work at all and men were expected to earn all of the money for themselves and their family.
We are not as far removed from those days as people would like to think, expecting the attitude to have completely gone away by now is very optimistic.
> Why is it acceptable
Why would it not be acceptable?
Even in a society with zero bias in favor of men in the workplace, it's still possible that men wind up earning more due to other factors than just bias.
If men are earning more than women doing the same job while all other factors are the same, then that's unfair.
If men are earning more because they are working more hours, or because they are taking more dangerous or undesirable jobs that women choose not to pursue, or if men are choosing higher paid industries more consistently, then that's not unfair. That's economics.
Obviously plenty of women earn more than plenty of men. They don't, as a rule, chose to marry those men.
Even in situations where people are paid by algorithms (i.e. only based on the work they perform, regardless of their sex), men earn more than women because they simply try to earn more (i.e. they optimise for earnings).
"Study of Uber drivers finds men earn more than women — for three key reasons"
https://www.latimes.com/business/la-fi-uber-gender-pay-20180...
The article above doesn't discuss it, but another reason is that men work more; so for the same hourly wage ("equal pay for equal work"), men would still earn more.
https://www.forbes.com/sites/karinagness/2016/06/30/new-repo...