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.
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.
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.
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)