I' even doubt the premise that there are terrible open source UIs created by designers. The vast majority of OSS GUI apps were probably styled by programmers, and I think they proved quite well over last 20 years that they chose well when not becoming designers.
The problem with design is also that it doesn't work very well in the "bazaar" style of development. Consistency is possibly the single most important feature of good design, and it gets hard to achieve without central authorities.
However, I don't think users care that much about consistency, unfortunately (even though I think it's important aspect of design). Look at web, every website is different and it's considered a good thing by designers. There is no consistency on web and yet desktop apps struggle to compete.
With GIMP it's the sliders in the tool options. Once you have them figured out (the trick is, they behave differently when you drag on the top half of the slider or the bottom half ... it's indicated by the cursor changing, but still) they're kind of usable, but still awkward. If you don't know how they work (and they're quite unlike any other GUI widget I've encountered), you click and now your brush size is 1233.12 pixels, so you just decide to enter the value manually by keyboard. There's a couple of truly useless default values for some tools. Third, if you want to adjust or create a new brush, or gradient, the workflow has about 2 popup dialogs too many. I could go on ...
In InkScape, my main beef is the gradient editor. The old one was awkward and counter-intuitive when adding new stops. Then they made some huge upgrades (honestly, a lot of great improvements btw), also changed the gradient editor. Now it's slightly less awkward and slightly less counter-intuitive, but not much.
These are two great pieces of software and I love using them for the most part (although GIMP really needs to grow up and support 16 bit colour), it's just that these are some really odd choices for a couple of UI widgets that get a serious amount of use in most workflows. And I don't understand why, they could literally implement a design based on any vaguely similar control widget in other software and it would be 10x easier to use. Yet they choose to roll their own very unique behaviour that honestly just doesn't work very smoothly.
I think the problem really is lack of humility of some designers, which was trend-set by Steve Jobs. SJ didn't do what users wanted, he had a very strong sense of his own aesthetics (lack of buttons, for instance). This IMHO works if you can create fashion trend around it, but otherwise, it leaves a lot of people (maybe even majority) in the cold.
He didn't have strong opinions on many things, he had a very compotent team and he continually pushed his teams to their limits.
iPhone was the result of a long and iterative design process involving many smart people.
Here is the ancestor of the iPhone from 1983: http://gizmodo.com/279688/apples-original-iphone-1983
Apple was working on the tablet project (iPad) long before they were working on the phone project. First iPhone designs indeed had physical buttons like iPods of the time. Then they decided to marry the phone project and the tablet project. Hence a phone with a multi-touch screen.
Of course all of these are marketed as if Steve Jobs was a genius and everything was so obvious to him from the beginning. That's just not true.
the fact that many of the developers are also users and as such are pretty opinionated while the designers that come to open source might often be very eager to "set their mark" on something?