The TI PRUs have decent tooling around them, if you use TI’s Code Composer Studio and toolchain. There’s even pretty good documentation on how to code for them. TI has been happy to offer training at my previous company when we asked about it.
I think it’s just that many people haven’t really used the PRU and the way the PRU works is different enough from a normal microcontroller that not very many people have decided to write “open/free” code or to write about the PRU in general on blogs. The PRU can solve very interesting problems basically for free (it comes in the SoC and often you can’t buy the SoCs without them), but many times there’s other easier to understand ways to also solve these problems.