UX people cannot make their own apps without hiring devs ( requiring money ).

Devs can create Open Source apps but they cannot afford to hire UX guys so the UX sucks.

Larger projects can attract both devs and UX guys but getting to popularity is hard and, even then, coordinating these diverse teams ( eg. dev and UX ) take project management skill and effort. That is yet another skill set that has to volunteer to be paid.

For a proprietary app, you simply hire the people you need. When you start, you need UX either to attract uses or money. So, UX is one of you first roles you spring for if you are proprietary ( depending on the market ).