I think about this quite often when creating new applications.

Four hypotheses why standard users are often the primary target of design:

(1) Power Users are louder but mostly ignored due statistically being not relevant.

(2) During design sessions the teams empathize with a standard user, not the power user. I've seen this pattern over and over again.

(3) Current web technology makes it difficult to build high density UIs that work well.

(4) Mobile UI first. If it works on mobile, we can just use the same UI for the web.

All this leads to another problem: Your standard users never become high power users on your platform. In the end platforms become interchangeable.