I've been an ad manager for three Fortune 100 corporations and I am also a professor in a media department and teach both graduate and undergraduate courses in advertising. I would say what MG says above but with a slightly different angle.
As she says, they are buying exposure. The advertiser is looking for a target audience and the target audience Stern delivers is not found many other places. They are hard to reach, so there isn't much competition for these advertisers to go to.
Second, I doubt if the advertisers even listen to these ads. They probably don't even know that Stern et al is even showing such contempt for the products that sponsor his show. After all, the advertising managers aren't in the same target group as the type of people who listen to his show.
You say that because he's enormously popular, any advertiser would want him to advertise their product. That would not be the case. I would say most advertisers would not want to identify their product with Howard Stern.
Now, I doubt you care this much but if you wanted to have a little fun with this, you can test out whether I am right or not. Record one of the commercials Stern delivers and send it to the advertising manager at the corporation (not the ad agency, the corporation). Tell him/her the type of thing you mention in your post, like about him saying he drinks wine only. Comment on the poor, unconvincing delivery. Then ask the ad manager why they put their money into such an unconvincing presentation. Say you are what you are--just a customer with curiosity.
I'll bet you anything Stern will change his delivery. He may even resentfully comment on what the sponsor conveyed.

Cher