From another hand there are thousands of ex-googlers left company to engage in startups, and Google likely doesn't have enough lawyers to go after each of them..
You're not arguing that it's legal; you're arguing that you're unlikely to get caught. That's different.
Left to join/found a startup is different from working simultaneously at both.
Yes, but Google need some resources to monitor and research who exactly left to found, and who was working simultaneously at both with conflict of company interests.