Because there is a pre-existing market with network effects. The one thing that Google just doesn’t 'get' are network effects.
Regarding Yelp!: Google has spent a lot of effort (leading even to confrontation in court) trying to kill Yelp!’s business already.
Google has network effects as well as they own Android dialer, hangouts, google messenger, and email. I don't buy this argument. They just failed.
Also you'd be the first person I'd see that defends Yelp's business model, but yeah I'm not familiar with the Google vs Yelp court case.
Google copied Yelp!’s ratings, showed them without any backlink in Maps and Places.
Once Yelp! complained, Google told them to either suck it up, or to use robots.txt to opt-out of Google search entirely.
"If you want to be in the index, you have to allow us to scrape your data and present it however we want".
This is a big part of the current EU Antitrust case, btw.