Because the odds of getting Famous Joe Blow X to run a HN AMA are much, much higher than getting him to become an active HNer. We're talking about a one-time commitment versus extended involvement in our community.
Look at the roster of AMA guests on Reddit. How many of them are active Redditors? It's non-zero, but not substantially so. And yet, by participating in AMAs, they enrich Reddit's community.
EDIT EDIT: removed some belly-aching