People have been talking about an ad inflation.
Showing one person 500 ads is just not going to make them buy 500 products. Out of all the ads a person sees, they'll maybe buy one product which they wouldn't buy anyways. And the value of that one product is the value those 500 ads have to share between each other.
So, individual sites showing more ads just means they get a bigger slice of that cake. Until the other sites increase how many ads they shove into our faces, too.
It ends up being an arms race and everyone loses.