It's easy to vent frustration by posting a forum message that says "I'm going to replace AdSense with direct ad sales--take that, Google!" Making good on that threat is harder than making the threat, especially in today's advertising economy.
True. I have some great numbers testing direct ads on my site vs adsense (clicks and performance) but is not easy to convince advertisers to begin posting their ads on your site... is not easy to prove them your performance.
It is easy for advertisers to trust a big G than a site they might not know or knot little about its performance.