The part about healthcare is USA-specific, but the relationship between total hours and total pay is nonlinear at other places, too.

In Slovakia, the healthcare is set up so that everyone pays a fixed fraction of their income, and then everyone receives exactly the same healthcare regardless of how much they paid. So it shouldn't have any impact on hourly rate.

Yet, it is difficult to find a part-time work on the market. When I tried it, I had to work for 50% of my previous salary just to reduce the work to 4 days a week, and the employer still believed they were doing me a favor. (After a few weeks I decided that getting 50% of money for 80% of time is not a smart deal, so I quit.)

I believe the problem is signalling. Almost everyone is okay with working full-time; especially men. (Women can use having small kids as an excuse for a part-time job, but that also dramatically reduces their hourly rate, which is an important part of the pay gap.) If you are a man unwilling to work full-time, it makes you weird.

So it's not like the employer literally needs you there 5 days a week. It's simply a decision to not hire a weirdo, when there are non-weird candidates available. If you differ from the majority by not willing to work 5 days a week, 8 hours a day, who knows what else is weird about you? Why take the unnecessary risk? Also, well-paid employees are supposed pretend they love their job; and by asking for a part-time job you show too clearly that you actually care about something else more.

Thus, I sometimes had jobs where I was able to spend up to 50% of my working time just browsing websites from the company computer. But no comparably well paid option where I could officially work 4 days a week, or 6 hours a day, and then simply go home.

(I was also trying to get home office, so that instead of browsing the web I could do something useful. But the companies where the employees spend much time online are usually on some level aware of what is happening, so they don't allow home office. As long as everyone must stay in the building the whole day, the management can keep pretending that people are actually working.)

I believe that if for example 50% of people working in some profession would demand part-time work, this problem would mostly disappear. Then, wanting to work part-time would simply be normal. But that's a coordination problem, and I don't even know how many people would actually be interested in working part-time if that would be a legitimate option (with the same hourly rate).