Most people use ad networks and ad networks generally don't provide a way to get the ads on the server side. That would also be more complicated than sticking an iframe/script tag on the page for the publisher.
If you don't want to use a network, you have to sell the ads direct yourself - which isn't easy.
I am aware that it would be more complicated, but not impossible. I guess for the time being the percentage of blocked ads is still small, but once it gets significant websites and ad providers will take the next step in the arms race
If you don't want to use a network, you have to sell the ads direct yourself - which isn't easy.