The academics in question might have little choice but to give away some/most of the content for free, as it is often mandated by university- or department-level policy. At least as far as internal use of the courses is concerned, the incentive here is clear for the university: reduce your dependency on a given superstar prof leaving (very slightly, admittedly).

As for the ego trip, I doubt it (in most cases): research gives you the recognition of your peers. There is a clear trade-off in allocating your time towards stellar teaching or stellar research. Few professors manage both. And stellar teaching tends to get much lower recognition from the peers.