I think the answer comes down to the fact that it would take, at minimum, one highly skilled audio engineer with specific audio programming skills to develop reliable variations that sound good. The sounds need to be realistic and appropriate literally every time, despite also being "random". This process would be deceptively hard, and take a significant amount of time.
How great or necessary is the payoff in most cases? For example, Dota 2 is widely regarded as having superb audio engineering - experienced players can tell what's going on even in frantic ten player battles involving 100+ different heroes and hundreds of abilities by ear alone. Dota doesn't use any procedural generation at all, and I've never heard anyone complain despite the thousands of hours they put in.
There is only so much you can do with pitch changes and creative use of filters, and having multiple samples for each sound can burn memory (and sound designer hours) fairly quickly. As you point out there is a wide variance in the technical skills of sound designers (some love playing with the game engine side, some are better at making sounds and throwing them over the wall).