Because search quality drives ads. There were search engines around but when Google came out with superior search results everyone switched and those search en

Because search quality drives ads. There were search engines around but when Google came out with superior search results everyone switched and those search engines quickly vanished. There are search engines in direct competition with Google today. If Google does not provide the best service there's an extremely low switching cost. Bing, Baidu, DuckDuckGo et al would be happy to take your traffic.