Building a basic search engine is relatively easy. Building one that rivals Google is extremely difficult, and not just because they're so big and convincing people to switch is hard. It's much easier to have good results when you know that the websites you're indexing don't care about you at all. Once you get popular enough to rival Google everyone and their mother will be trying to game you and that changes the problem significantly.

The original implementation of the Google search engine would get obliterated today, though I guess you have to start somewhere.

There's another incumbent advantage here: I imagine it's much easier to provide good results when you also have data on which results thousands or millions of people clicked on for millions of search terms.