OS widget libraries aren't always big enough to solve all problems. On the web, there are many frameworks that provide widgets for typical use cases.
But even if you have a library with hundreds of widgets, you can still make a terrible UX if you don't understand good design, and many programmers don't.
In my experience most designers don't know what UX is. They think their job is to make it look pretty. If it needs 3x more clicks to do the same thing as before so be it.
