There is a shelf-space issue.

Let's assume most of high-quality ad-spaces are available for "broader campaigns". Once each small business broadcasts ads here, they will compete for space. Price of space-unit goes up.

Then some ad-space-seller opens Pandora's box: "hey I can allow you to narrow the audience by location targeting. Each unit is more expensive but conversion rate is higher so you will end up with a better margin overall"