[Plyphon_]

It's a good question - and the simple answer is -

Usability and interaction design is most certainly part of UX. However, the skills needed to be a competent visual designer match (in terms of difficulty, time to execute, etc) that of being a UX designer. Simply put, in order to go deep enough on either craft, you need to dedicate to one side or another.

There are many dual 'UX/UI' roles out there, but they tend to be weighted toward the UI side, maybe with some user testing tacked on the end or some journey work.

Sole UX roles are much deeper in their reach within an organisation and consist of much, much more than just wireframing and making visual deliverables.