> What exactly is keeping developers from making a fully featured ... web browser ... ?

I think the biggest issue is the complexity and scope of work. If you want a web browser, I can make you one over the weekend, it just will only work with a tiny subset of pages. To fully handle every website is so hard even Mozilla gave up on their rewrite of their engine, Servo. If it was easy, they probably would have finished.

If you are suggesting, "Why don't we all pay a bunch of progammers to make the world's best browser that is everything I want?", you can try, but everyone has different ideals about what a browser should be.