Simple.
Because if they did assist with FOSS projects much of the time they would be dealing with developers who (a) think they know what users want better (b) have no respect for designers and (c) believe obscure feature X is more important than UI.
Remember this is the same group of people e.g. on Slashdot/Reddit who think the iPad is an overpriced toy and think people are sheep for buying it.
Within a single release cycle, these designers managed to destroy what was once among the most vibrant of all open source projects. The existing GNOME users have been alienated by the absolutely horrible UI. Among their ranks were many good developers and other contributors, too, whose absence now prevents the project from recovering. A large portion of these users moved on to KDE or Xfce, rather than MATE, causing further harm to the GNOME project and community.
There are other cases, too. Windows 8 and the UI changes starting with Firefox 4 are other recent, and major, examples of the work of designers going terribly wrong. The end result is software that is nearly unusable, or much less usable than previous versions.
The skepticism you describe is very well earned and deserved.
If you could give people a completely fresh, unprejudiced choice between Gnome 2 & 3, Windows 7 & 8, or Firefox 3.5 & 18, I bet many of them would choose the newer version. I don't think (most of) these UI decisions are objectively bad, and over the long run they might even pay off. But when the user upgrades and suddenly can't see how to make it Frobnicate, they get angry.
For my personal use case for LibreOffice specifically, I don't give a damn how ugly it is, and I'm willing to have some patience if it's a bit slow, but crashing or hanging on a 160-page document is unacceptable. So adding a designer to the project wouldn't add any value to me.
But FFS why did you have to bring apple in to this. Absolutely nothing to do with this, quit it.
That's because it looks like everything designers are touching (Gnome3, Unity and partly Firefox) starts to turn to crap.
Yes, I completely respect Designers and I understand that they can do wonders for every FLOSS project and I know that this sentence is completely biased...but there are many examples out there were a designer (or multiple) are trying to stuff their vision down our throats without any afterthoughts.
No, actually he made a very good point by bringing Apple into it. Apple, see, is the very place were design and UX reigns supreme over other considerations (of course they also have faults, every team has).
So, OSS developers bashing Apple and its UIs, are not the very best community to nurture biding UI designers.