As someone who's almost gotten their PhD in (mathematical models of) medical imaging, I think you'd be surprised at what kind of work would be consistently challenging enough to interest me.
I was at a job forum a couple weeks ago, and whenever I would bring up my incoming doctorate, company reps would be like "Oh, well our stuff must be pretty boring to you, then." I really wanted to say, "If I'm talking to you, it means what you're doing is interesting to me!"
I'm sure some PhDs are driven to improve the state of the art, but there's also a perception that a PhD won't want to go back to working on problems that are already solved in principle -- and that perception scares me, because I'm going to need a job soon....