Here, it's for the oldest of reasons. Deans get their choice of the prettiest maidens or most strapping lads in the college, plus a good parking space.
If only (I don't get a parking space)..
OK, seriously ... when I was a faculty member, I had direct influence over the students in my classes and my lab; my ability to have a large positive impact was somewhat limited. When I was a Chair, I could lead disciplinary curricular reform to improve student opportunities and success, and gather resources for faculty who were better at teaching and research than I was.
As Dean I am able to do all that, but also allocate broader college-wide resources to buy faculty stuff, send them on trips, support student research, etc. If I think about the influence I have had overall, I have done a MUCH better job in getting the resources for other people, than from working on my own teaching and research.
I actually enjoy doing a lot of the stuff that many faculty hate doing - and by my doing it, they can move the institution forward through their individual efforts.
this sums up quite well why i became a dean. i can make a difference on so many people and on the University, far far far more than i ever could as a member of the faculty. as a faculty member it was often all about my research, my classes, my students....maybe....my department. as a dean i see a much bigger world and i enjoy it much more.
by the way, money is not the reason someone should become a dean. a desire to make their school better is.