Software implementations of those features is often slower, and runtime checking can often be too expensive compared to the gains.
since it seems like nobody but cachy or custom kernel runs anything but V1
Gentoo offers x86_64-v4 binary builds too.
There was a proposal for Fedora too, though it was ultimately rejected.
Ultimately the gains right now seem to be only 1-3%, though that might also be because there’s not much demand for these kind of optimizations. If more distros enable them they might become more widespread and the benefits might increase too. It’s a chicken-egg problem though.