Just because it's Google's code on the publisher page, doesn't mean it's Google's customer's ad that shows up on the page. It's entirely possible a third party is willing to pay more than any of Google's own customers, so it's auctioned off to Google's customers, and Google's partners (who auction it among their own customers).

Also advertisers often want to do dynamic stuff too. Or may be willing to pay more for the same user in different contexts. Or utterly unwilling to have their ad on sites with UGC. And you don't know where the user will show up next.