This is the lion's share of the problem, I suspect. OSS projects are driven by the people who write the code, and they don't take orders, and they don't massively value making things easy to use - or even necessarily have much connection with what that means. Current computing is such a pain in the ass that we've basically selected for inhuman levels of persistence and patience, and placed a large cultural value on those traits to boot. "Read the docs", we say. "Use the source", we say.
We don't have any idea how many human factors experts are out there, willing to help, because there's no process for them to contribute beyond "submit a pull request" - and even then, a purely UI tweak will meet with huge resistance. The UI expert is going to have to be a diplomat as well.
Bad UX is subjective. Money doesn't change that.
Suppose we want to optimize user productivity. For any given UX, some users will feel they're more productive with it, and some less. Finding a UX that maximizes the number of users who feel productive is going to help attract users to the product, and money can buy the design expertise needed to find that UX.
It might be the case that users only feel more productive with a given UX and aren't actually more productive with it. Even then, if a majority of potential users think that the UX is making it easier to accomplish their goals, a project will have an easier time attracting users.
I agree that many FSF/GNU projects have made horrible technical choices, but I don’t think that’s really the root cause here. The reality is that good software requires an enormous amount of skilled labor to create and maintain, and the FSF and other OSS orgs can only muster a small fraction of the engineering hours that a mid-sized for-profit software company could.
In addition, good user-facing software requires more than just good engineering - you need good UX, product direction, etc. Sometimes the creator or maintainer of an OSS program will have the good fortune to be reasonably good at all of these, but it’s very rare, and there aren’t nearly as many UX designers or product managers looking to contribute to open source as there are developers. Even if someone did want to help in this role, the developers on most projects would probably ignore them.