It's _vastly_ harder to get ahead on the "engineering" track. At Google, for instance, it's to the point where as a manager you barely need to have a pulse to get promoted (as long as your reports are any good), whereas on the technical ladder promotions to Staff level and above are stupid hard and downright nondeterministic.

That's why many engineers there, who have little to no leadership skills or inclinations, end up being managers: it's simply much easier to get ahead on the management track, comparatively speaking. There are simply a lot more senior managers than there are comparatively senior (by comp, the only metric that really matters) engineers, and you aren't competing against as strong a set of peers.