The "many" examples you gave is actually just one: Microsoft. Both Yahoo and DuckDuckGo searches are essentially based on Bing.

There is tremendous barrier to entry in the search space - otherwise Bing wouldn't still suck compared to Google. The point of competition is to get good enough to steal market share from the entrenched incumbents or gain new users somehow. If that doesn't happen, for whatever reason, then you don't have strong competition.