exactly. what we are talking about here is balance. as intermediary, they have to care for every party to keep growing and in order to justify themselves before their shareholders.
similar business models aren't nearly as successful, because of a lack of balance: huge incentives for the one side and minimum earnings/high risk for the other side don't attract enough participants for a healty marketplace.
for sure, every side prefers secure earnings, low risk and pushing off labour to the others - but that is hardly possible to achieve for everyone. what we need is market balance.
in this context, i want you to understand that cpc is the base of googles' success story. no other accounting method would have been capable of attracting both parties in crowds. of course it took a big company to push this through on a market that was dominated by lousy affiliate programs and poor business prospects for content publishers. one cannot disqualify cpm and cpa, but they are and will rather be admixed by-products, whereas cpc likely stays the core product providing the highest total utility.