Earlier quoted context omitted.
Search, and thus Google, obviously has different economics than something like eBay: it's very easy to switch to a new search engine, and there aren't really demand-side economies of scale like there are with auctions. eBay may not have been the first auction site, but it was the first to hit a critical size, beyond which the advantage was all theirs.
Google makes money from selling ads. Advertisers want to place ads on the largest search engine sites (or one that can deliver a target audience at an effective price). It may be easy for a user to switch (although letting users know that you exist can be hard/expensive), it's less clear that advertisers will switch to a search engine with few users. Google's customers are the advertisers.
Technically. They do a great job of monetising search traffic but that's not the bottleneck.
Search engines win by attracting their customers' customers, users. Advertisers will follow the users.