Cheap content attracts cheap commenters. The comments and questions in AMAs rarely express novel insight -- usually, someone pipes up with a popular sentiment, like asking President Obama to stop using drone strikes. Commenting in such threads, which are inevitably mobbed, gives you the illusion of significance ("Wow, X answered my question!") or worse yet they give you actual significance ("Wow, it's X, he did an AMA") both of which function as an invitation to horse-sized-duck trolling and celebrity obsession. Do we want the users who join because Tim Cook did an AMA here once, or people who join to discuss hacking?
As the old saying goes: small minds discuss people, average minds discuss events, great minds discuss ideas. HN has always been a place for great minds -- let's keep it that way.