Engineers at Google are not hired for specific projects. You're appraised as a good engineer, therefore we want you at Google. If you're not happy doing what you're doing, we'd like to find something you will be happy doing. But that doesn't mean you can just chop and change projects at will. It's considered pretty uncool to just quit a project abruptly without making the appropriate arrangements first. You've got to leave your work in a good state so that your teammates and successors can pick up where you left off.
I don't know anything about Steve's situation, but I would assume he did these things.