I sort of understand why search is a near-monopoly: it's free, and there's somewhat of a network effect and a self-reinforcing credibility thing. But what are t

I sort of understand why search is a near-monopoly: it's free, and there's somewhat of a network effect and a self-reinforcing credibility thing. But what are the barriers for competition to AdSense? Why isn't some other company offering an alternative, like the same thing at a higher price with better support?