There is a point, once you have played an instrument long enough, that watching other people play becomes challenging. Actually, to be perfectly honest, it goes both ways. Watching most people is a challenge while watching a few becomes extraordinary.
I think a lot of it is that the mystery is gone. You understand how everything works, you know how you would approach it, and you are looking for that person to either make the logical choice, or completely surprise you. When something isn’t as profound as you could imagine it being, you get frustrated. When something is derivative, or too easily come to, or too obvious, you are upset, and when something is poorly executed you are furious.
I imagine the experience is similar for anyone who has practiced practically anything for long enough.