He probably wanted to participate in growing another new tech company that has a lot of potential. That's an opportunity Google would find hard to counter.
On the issue of challenges, maybe Guido has more pull on this, but there were recently some articles/discussion on HN about Google's hiring practices. They might hire you based on your PhD in databases, and then have you writing shell scripts for work because some randomized 'sorting hat' tells you what group you end up in. It's possible that even Guido doesn't have enough pull at Google to overcome this. :-\
The problem with new hires is that no other team has any idea whether you're any good, so it take a bit of tenure to prove yourself and make yourself known to others.
I'm quite sure Guido would be doing whatever he wanted at Google
E.g. if Guido ends up writing shell scripts, but his real skill-set/passion is for databases. It's quite possible for him to under-perform, and then that performance used as a reason for the 'database team' to not take him.
I have enough pull at Google to overcome this, so I'd be pretty surprised if Guido doesn't.
There are also things related to company size. I am more confortable working for small companies than big ones, there are other people that is the other way around...
It's not as simple as saying: "please stay, whatever you want"