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Most medium to large media companies do have their own sales staff for online advertising! The problem generally is that they can't sell enough local/direct-sold ads at a high enough CPM to sell out their inventory. This is where they use something like AdSense, to fill in those gaps where they couldn't sell anything for a higher rate.

For the same reason why newspaper ads are rarer these days I would guess - next to nobody goes to them for their ads so the prices demandable tend to be lower than commodity or harder to fufill.

I can see the cost of sales being quite high.