If Google just doesn't want unionized engineers, it could just fire all engineers. Obviously they need to balance their various objectives and firing productive people is costly to the business. I think they are hoping more to send a strong message to the whole company with as small a disturbance to the business as possible.

Because reviewers aren't held to the same level of responsibility for the content of the change as the author. Making them responsible for back-door vulnerabilities slipped in by employees compromised by a nation-state would grind the company to a halt.