I agree. Although there are millions of potential readers, only a very small number of them ever learn about our books' existence through discount newsletters, which ironically are our simplest and most reliable method of advertising. Selling the same book to that small group over and over has diminishing returns.
Amazon's store literally has more books than any one person can even look at. We don't have to buy the books we've already passed over once, no matter what the theory is about the number of impressions before someone buys. Multiple impressions may waste my time by attracting me with beautiful cover art, but I do my research before buying. The cover may scream "Good fit!" but other elements tell me "Nope!"
I see the same books get advertised again and again in the discount newsletters, usually about a year apart. I pass them by.
When my AMS ads worked, my books were being shown to people who hadn't seen them before. Big difference compared to the discount newsletters.