@mcc @AlgoCompSynth @bagder It also has not been shown to increase revenue or decrease costs (for the advertisers, obviously the ad placement networks get money).

Worse, at least some common advertising techniques have been shown to not increase revenue. The canonical example here is a study run on eBay buying search keywords, which found that when they didn't pay to place first in the search results literally everyone who clicked the first (non-eBay) result immediately returned to the search page and clicked on the actual eBay website.

This did not change their search-result buying behaviour.