Since the Internet is so huge and disorganised, big advertisers tend to use agencies to place their ads for them. The agencies find it more expedient to put their clients' ads on trusted, big brand sites. Even the ad agencies whose job it must surely bo to do this sort of ad placement are not likely to take the time. In other words, most of the big banner ad money is going to the big sites. You can probably get some agency to sign you up but they are unlikely to do you much good.
Agencies also have more pages to sell than they have advertisers for.
They're going to place the ads on the biggest and best sites. The ones with the big brand names. Yahoo, for example, sells out less than half their inventory and they sell cheap. Yahoo is not someone it makes sense to compete with in most cases. Although there may be widget sites out there actually making plenty of money selling banner ads it is an uphill battle.