Maybe. I daresay you don't know that and neither do I. You suitably chastize people for presenting speculation as fact, and I think you're doing the same thing here.
One thing you don't mention is that those "sites" do have to undergo a review to recieve ads from the major ad agencies, and my experience is that places like burst, casale and fastclick, to name three, have much more careful site reviews than adsense.
Could the junk sites get in? Maybe. Maybe not. And if not, what return do they get on the barrel scraping cpm ads?
It's my opinion that adsense has been a huge factor in encourage people to generate junk sites.
Right now adsense gives junk site owners access to monetizing a huge range of subjects, and the money is clearly superior to low paying cpm ads, or affliate programs.
And some of the junk sites created for that purpose end up in the SERPS.
On the adsense side, the solution is obvious and has always been obvious, except that google refuses to ENFORCE ITS TERMS OF SERVICE. And that applies to both adsense and adwords.
Will it solve the problem of spam? No. But it's an obvious step. Or simply give up -- change the TOS and take everybody, black hat or not. Junk or not.
This half way in the middle of enforcing its own rules is making things worse.
PS. To get a sense of the effects of adsense, one can read the messages on this webmasterworld. How many of the messages either outright display a "don't care about the rules", a "don't know what the rules are", a "how do I beat this rule?", or suggest that there are a whole lot of people with limited ability to create good content, and simply want to hop the gravy train.
So long as google let's them, they obviously have the right to try (at least at their own risk, if any). But it doesn't change the consequences.