I've actually worked with a genuine UX/UI designer (not a mere Graphics Designer but their version of a Senior Developer-Designer/Technical-Architect).
Lets just say most developers aren't at all good at user interface design.
I would even go as far as saying most Graphics Designers aren't all that good at user interface design.
Certain that explains a lot the shit user interface design out there, same as the "quality" of most common Frameworks and Libraries out there (such as from the likes of Google) can be explained by them not actually having people with real world Technical Architect level or even Senior Designer-Developer experience overseeing the design of Frameworks and Libraries for 3rd party use.