Seems like the question would be to ask why nobody competes viably with Photoshop. Premiere is an odd example to single out — Avid and Final
Cut, for instance, are full-featured and both have a substantial user base. The competition there is healthier than OP makes it out to be.
As for Photoshop, once it became the standard, it stayed that way by virtue of user inertia and the massive development resources it would take for a competitor to create a program in the same class. Photoshop is an extremely complex app and, on the whole, a very good one. It’s not without its frustrations, but nothing is.