I am not saying that the role of a senior engineer and a staff engineer is the same. I am saying that the primary --if not the only-- reason people get promoted

I am not saying that the role of a senior engineer and a staff engineer is the same. I am saying that the primary --if not the only-- reason people get promoted is because management figures that the cost of paying them more is less than the risk of them leaving the team. And, indeed, I believe that applies to every promotion up to something like the C-suite, where I have zero experience and can't comment.