a) the roles are fewer
b) companies are wary of bringing in an EM for a team (prefer to promote within the ranks, in my experience).
c) switching from EM to a senior engineer role is hard both because it feels like a downgrade and because I see companies not that willing to offer it.
Of course, if the company is on the right course, you've hit a jackpot and can become an important leader few years down the line, but I know many EMs who are trapped in a bad org because of this. I believe engineers transitioning to a management role should keep these things in mind. I prefer making less bucks and keep my peace of mind about moving somewhere else.