With the very low income, they have to because there’s no room for error and low enough on the income tree, it’s a real financial loss plus massive stressor to have two workers maxing out at 43k or so.
For the very high income (in W2 income terms anyhow), it’s related. If your husband makes 400k, you get the same benefit spending 10 hours a week or even month finding an extra 25k in the budget as you would working a 50k/yr job because you only end up with a little more and you have to work 40 hours a week to get it. You have to crack six figures yourself before the extra money is harder to find via frugality than just working a job for it.
This isn’t to say that frugality is pointless unless you only make under 40k or over 400k, but that at the extreme ends of wage income (as reflected in both extremes having the highest rates of SAHMs), it’s mostly going to be easier to conserve cash rather than earn marginally more cash.
The math is different closer to the median married income, which is partly why the median is rising. The reason is that people who are willing to marry when both incomes are likely to be about even set up their finances differently and as a result losing one income doesn’t create the space to segue into conserving the remaining one.
Of course, another reason the median married income is rising is that if you weren’t taught household management and homemaking skills, which is a very large number of marriageable women these days, it is terrifying to figure out how to get along on a low income and marrying a higher earning man sounds like it will be safer/easier.