Simple answer: because they are lazy and they want a cheapest way, and ad giants like Google convinced advertisers this is the only way. Of course it's not, and

Simple answer: because they are lazy and they want a cheapest way, and ad giants like Google convinced advertisers this is the only way. Of course it's not, and ad deals with no middlement can be extremely efficient and financially advantageous to both parties. Not to mention they're often adblock-friendly because of being served locally.