There is one simple reason that I'm leaving academia: I simply love work I'm doing at Google. I get to hack all day, working on problems that are orders of magnitude larger and more interesting than I can work on at any university. That is really hard to beat, and is worth more to me than having "Prof." in front of my name, or a big office, or even permanent employment. In many ways, working at Google is realizing the dream I've had of building big systems my entire career.
A big reason for this is the amount of time academics spend doing things that aren't teaching or research:
The biggest surprise is how much time I have to spend getting funding for my research. Although it varies a lot, I guess that I spent about 40% of my time chasing after funding, either directly (writing grant proposals) or indirectly (visiting companies, giving talks, building relationships). It is a huge investment of time that does not always contribute directly to your research agenda -- just something you have to do to keep the wheels turning.
There are good reasons to be an academic, but it is not for everyone.