Not a downvoter, but I suspect it's because we all know that people have a tendency to act in their own best interest.
But, we also expect leaders to consider the best interests of their charges, as a function of good leadership.
I see. It would be nice, but leadership like that are few and far between, and they probably get it beaten out of them. I don't think that's even taught at MBA school. When I started all this, someone told me, "they show appreciation through your paycheck." Not very satisfying, but there it is. If you're productive, they certainly miss you when you are gone.