Why do most popular adnetworks prefer to charge CPC or CPM, instead of CPA?

I can't speak for specific networks as that is a broader strategy decision they have specifically made to position themselves in the market.
That said, we need to have a bit of a history lesson.

It kind of all started when Google decided to introduce this little model called CPC or Cost Per Click. The reason they did it was because advertisers were sick of paying for wasted impressions and wanted something a bit more performance oriented to mitigate some of their risk.

Risk is an important...

Why do most popular adnetworks prefer to charge CPC or CPM, instead of CPA? WordPress ...