CPC is actually only smallish percentage of the online ad marketplace. The real dollars are in CPM ads - display / brand advertising, the stuff you see in the big skyscrapers and 300x400s on high-traffic pages.
It's about brand awareness, not clicks or conversions.
Those placements run at pre-negotiated rates, and represent the bulk of ad revenue.
"How do I trust this 'Buzz Cola' who have spent .01 cents for a 4 inch ad on MY facebook page?"
...people don't get a sense that the ad campaign is city-wide, nation-wide, international, whatever, and is being shown to millions of people...even when it is. And so the brand-building ("everybody's doing it!") doesn't work the same at a psychological level.
I'm not suggesting that there is actual ratiocination going on when people look at ads. But a large, public, showy ad argues from authority much more than a little flickering thing on a website (that only I can see).