My thinking was, if anyone could get pregnant, we might see the idea in the workplace that "well, most of us are likely to need this consideration at some point, so it's overall fair to provide it to those who need it." But that logic doesn't hold in other cases that are nearly universal.
Bereavement, for example. Probably nearly everyone will experience a devastating loss of a dearly beloved person during their working years. People grieve differently - and a few will want to continue on at work as if nothing as changed as part of denial - but most aren't going to be worth a whole lot in the office/assembly line/cash register for at least a few days. And it would be nice, when our turns came, if we had a few days to take care of all of the bureacratic paperwork that goes along with death.
I was amazed to learn that the Dear Spouse's company offered bereavement leave. It was only a day or two, but I had never heard of such a thing. Additional paid leave for bereavement - and they let him take it to support me, when we weren't even engaged! Wow! ...My reaction suggests to me that, even though you might think many companies would offer "compassionate leave," on the "I'll need it one day, too" principle, most don't.
I believe FMLA covers bereavement (for companies of over 50 people, if you've been there longer than a year), but makes no provision for paid leave. They just can't fire you while you're gone. If you work at a small company - say you're a shift worker at a franchise operation - and you say you just can't handle work this week, they can say, "Then you can reapply for a job when you're ready to come back, because we're firing you."
I have no background in labor history, but I bet it's interesting. It must encode all kinds of assumptions we have about work, workers, and working and how they've changed and are changing.