I’ve often wondered this as well. My conclusion, after not thinking too deeply about the issue, was that it’s a combination of the difficulty in match making and poor tooling. (There are many startups trying to solve those problems, but it doesn’t seem like anybody has cracked the nut yet).
There’s lots of wisdom in this post by Curt Corginia:
A wise, mature person would treat the software engineer interview process as a pure learning experience. He, or she, would enjoy learning about companies out there for the sake of research, interacting with key players, and mastering the art of whiteboarding. It would just be like a fun game.
I don’t think of it like that, but a mature person would. Do what I say, not what I do.