"Accusation of sexual harassment" is kind of an understatement given the massive amount of misconduct that occurred, often not even denied by the bad actor, in direct violation of company guidelines. You don't just get to light the employee handbook on fire and keep all your compensation. You can get fired with cause.
Naturally trying to kick someone out and withhold compensation is a lot harder than just quietly ushering them out the door and paying them off, which is what typically happens here because Google does not want bad press and does not want to get dragged into a multi-year court case by someone they're trying to fire.