I'm convinced a primary motivation for hiring in mega-tech companies is to keep people from making startups to dethrone them. You can't legally compete with Google, etc if you've signed a non-compete.
A founder of a now-popular startup worked for Google eight years ago, so what? Let's say Google remembers and decides to pursue action. They'd spend another 3-4 years suing a person who has a mountain of plausible deniability to hide behind. And for what? There's no real gain to Google for this.
in this hypothesis most people working for google are actually not needed, but employed there primarily to waste their time