The problem isn't that Google just says, "Hi, donate time to us, your corporate overlords, in exchange for nothing." Instead, they make a very reasonable argument of, "You should help your teammates and colleagues."
Imagine that you're on a team with a junior developer. Their feedback is basically worthless to promotion committees because they're low-level. If they say, "Hey, can you help me understand why my code is crashing?" the selfish answer is, "No, if I spent two hours helping you, the benefit I provide to you is not measurable or quantifiable and won't help my case for promotion." But it would also be kind of miserable for everyone if you constantly avoid helping your teammates and only do the absolute minimum required to avoid standing out as egregiously unhelpful.