The same reason craigslist classified haven't been surpassed by a more modern system -- it is much easier for everyone to live with the shortcomings (which really aren't that bad) than it would be to recreate an equivalently large, diverse and vibrant community on another site.
Especially since any focused site that came up to do this would almost certainly put way more barriers to entry into the community than reddit (which still allows you to create a new account virtually instantly, with no email verification) has.
I think what he meant is that if a girl is giving an AMA she gets a lot of off topic "wow you're hot" type posts from the peanut gallery, something that wouldn't happen if the system were more moderated. OTOH, the lack of moderation is part of the charm of AMAs IMO even though it does result in the trolls being trolls.
(The "Woody Harrelson"/Rampart AMAs would be the norm if not for the chaotic nature of reddit).
I'm not suggesting they are all charming, some of them are pretty terrible, but that's the price you pay for having relatively free speech. While moderating the questions would eliminate posts like the one you linked, it would also eliminate ones which aren't terrible but are certainly "off the script" of what would be asked if everything were cleanly moderated.
And if it's a trade-off where more "impulsive" comments are deleted, then I prefer that to subjecting female respondents to constant come-ons from pubescent, sad human beings.
It seems that Emmy Rossum prefered Reddit's version, not the version you prefer.
Emmy Rossum went to reddit for an AMA, and used the site's commenting functionality to ask people not to be rude. She did not cancel her AMA, ask to have the rude comments deleted, etc.
I have the "Reddit Enhancement Suite" extension installed, and it allows me to jump to each comment he responds to. If you are a frequent redditor, I recommend installing it.
Why hasn't reddit's AMA been surpassed by a dedicated AMA site with a user interface designed for AMA?