Firstly, unlike backend/system/OS/desktop components where lots of people can write little bits and bring them together to form a whole, you have to consider the whole UI from the start and create a consistent interaction model and look and feel, probably with some visionary type figure wearing a nice scarf who is leading a small team of trendy designers and UX people. These kinds of people tend to be relatively rare in OSS, as undertaking initial design and then supervising a whole project through to "completion" is a sizeable undertaking and not really "scratching an itch" as many OSS projects start off as.

Secondly, the UI is where all the components come together. If you're making something that uses components A, B & C and when you use them all together there's some funny race condition or you don't quite have all the right APIs to make it all work together in the way that was envisaged, you're the one doing the sometimes thankless task of delving into all the bits and seeing what's broken where and how you can fix it.

Also you're the one looking at performance and final testing of the whole system - UI, middleware and firmware together. Two apps open at the same time cause too many wakeups and kill the battery? You need to instrument and fix it. Using the browser while crossing an international border and starting roaming kills the data connection until reboot? Sucks to be you - fix it.

Yes it's probably easy to lash together some kind of UI which allows you to "use" various features you'd expect from a mobile. It's very hard to make a "product" which isn't a complete inconsistent mess full of weird edge-cases that might be somewhere approaching usable as a daily driver or sell-able as a product.

It's taken Jolla, what, 3 years to get where we are now? And most of their staff are ex-Nokia so will have done and launched a similarly complex UI before on the N900 and N9. A good-quality UI for a consumer product takes a surprising amount of effort.

And as we've been discussing here, until you have something "nice" that people enjoy using, you're very unlikely to reach that critical mass where some of your users are developers too and they see value in jumping on board and adding stuff (in fitting with the OS design language too)