A big part of the value to the tenured professor is not having to teach an extra couple of sections of CS 101. The other part is plastering their name on a bunch of extra papers to game their h-index :D

I don't want to be too flippant here, but senior engineers generally gain very little from training juniors, not least because promo processes suck at basically every tech firm, and so the moment you train up a junior, they are going to leap to another company rather than go through promo...