I think that is the deal, explicit or implicit, in a lot of households - whoever earns more keeps going to earn more, and the other one becomes the primary carer, with all the career impact that involves. Men tend to be a little older and tend to choose higher-paying jobs so there's a definite vicious cycle aspect to it.
It may make financial sense at face value to allow the higher-earner free rain to push ahead while the other hangs back and becomes the primary carer, but there are all sorts of other considerations and knock on effects that really should be discussed and negotiated (not least, the primary carer's longer term career prospects/fulfillment and earning power).
I know a couple where one is a partner at a Big 4 firm, and the other reasonably senior in government, part-time. She (the latter) will always defend the value of her work against her partner's, even if it is lower paid.
In my case, I was just as ambitious and high-achieving as my partner but he ended up in finance and I ended up not, so his earning were always going to beat mine. We did 50/50 for a good long while (maybe even higher than 50 for him, because he's naturally very organised and attentive to domestic detail though of course I took mat leaves and endured the physical toll). But eventually I've had to fully embrace the primary carer role as his earning power and achievement have outpaced mine, which have sort of stalled due to my industry (shrinking), seniority and personal situation. I could have persisted in plowing ahead with my career but it would have caused so much stress and tension on the home front it wouldn't have been tenable.
Interesting article of relevance in the FT (to which I subscribe so someone should be able to read this link), by a male author no less: "What's driving the rich world's falling fertility"
With a few happy exceptions, maternity is a terrible deal, economically speaking, for working women. Although there is a regrettable paucity of high-quality studies on this, I suspect the pattern we see in the UK data would be even starker if we separated the reproductive choices of women who are the family’s second earner from those who are the first. The rise of so-called “greedy jobs” — where your progress is closely linked to how much of your personal life you are willing to put on hold — is a further disincentive to have children.
https://on.ft.com/3spO8Lx