Maybe network effect isn't the right way to describe it but clearly Google has a ton of inertia here since their company is literally the verb for search.

I agree that Google has a lot of inertia in both search and search advertising. That still doesn’t answer why there aren’t more well-capitalized companies trying to win the search market. I suspect (but am not sure that) the answer is the market is more visible than it is worthwhile (somewhat akin to media and entertainment businesses).