"One or two hybrid days per week capture nearly all the fertility upside." That is an interesting point, and not obvious why it would be so. In fact, it kind of calls into question whether the whole relationship is causal. The people who were able to WFH longer were more often in high-income jobs (service workers never got to do it in the first place, it was almost entirely an office worker thing). They were thus mor…
The other reasons given make sense to me, but I bet there is also some psychological benefit in having a regularly scheduled escape from home, and having a guilt-free excuse for it built in, which partly compensates for being forced to come in a few days a week. The contrast makes it easier to appreciate the company of your spouse and probably makes child-rearing seem less oppressive. People theoretically could manage this without work imposing it on them, but in practice, having to make and justify the choice creates stress.