Exactly that, the market is skewed by monopolies. You can easily setup an ad server, but good luck getting the attention of an advertiser. Google has a stronghold on display ads, almost a monopoly.
That's a demand problem not a monopoly problem. You can't get advertiser attention because advertisers have limited interest in advertising on small scale properties because it's not a scalable way to do business.
If anything Google provides more opportunity because the alternative to AdSense existing would probably be no revenue at all for small web properties.
