A couple things:

1. It facilitates a world in which only large content providers, who can afford to individually sell ads to advertisers, to exist.

There's a lot of overhead to ad sales and individual companies do not want to work with 1,000,000 providers, they want to work with 10-100.

2. It's substantially less efficient and only works for brand advertising or mass-market direct-response advertising.

One of the greatest things that Facebook and ad retargeting enabled was the rise of direct-response brands. Previously if you were selling a niche product - and most larger brands started out with a niche product - it was very difficult to reach an early audience who would be interested in purchasing your product. Facebook and Google flipped this on it's head, enabling millions of businesses to more efficiently reach customers. Facebook alone made the direct-to-consumer brand explosion we've seen over the last 10 years possible.