Why Google Only offers CPA program to limited Publishers

How many people in this forum are aware that Google has a version of AdWords that is CPA based rather than CPC based? Apparently it has been around since 2006 and is still in Beta. I just received my invitation to join the beta test, and it got me thinking. Here we have one of the largest advertising networks in the world, and there standard method of operation is all CPC based. If you think about the model, Google is basically the publisher and the ad network. And, it is obviously in there best interest to work in a CPC model.

So, the question is this. Why are so many affiliate programs paid out on CPA basis rather than CPC (or even CPM)? If CPC is still good enough for Google, shouldn't it be good enough for the rest of us?