I think it's partly because Sphinx is clunky and people have been using MkDocs instead, which I believe either doesn't generate anything from docstrings at all, or didn't until recently. I don't know why they're not using Pdoc, maybe they don't know about it.
I also get the sense that there is a growing movement among programmers that API reference documentation is useless, or not worth putting effort into, because it's possible to go out of sync with the code and "nobody reads that anyway". I've heard people say things like "if you're advanced enough to read reference documentation, you're better off just reading the source anyway." It seems to be rooted in some kind of combination of extreme cynicism, a distorted sense of how people who aren't raw beginners learn to do things, and of a kind of false minimalism, wherein API documentation is old fuddy-duddy stuff for Java and C++ developers, and the new friendly easy way is to just read the examples. Such libraries also seem to exhibit a poor separation of public and private interfaces, so maybe it's just a reflection of people being bad at designing libraries.