Not all advertisers believe in "smart pricing" or want to entrust control to an algorithm. That's especially true of mainstream advertisers, ad agencies, and direct-marketing agencies, which are used to targeting their audiences through careful selection of media or lists.
Don't forget, traditional e-commerce or affiliate ROI is just one measure of success. Advertisers in many businesses don't expect immediate transactions; they're looking for quality leads.
ADDENDUM: If Google wanted to offer tiers ("AdSense Premium") or other subsets of the publisher network ("AdSense Editorial," "AdSense E-Commerce," "AdSense Community"), it could charge a non-refundable application fee to publishers who wanted to participate. This would help to underwrite the cost of editorial reviews, and--just as important--it would virtually eliminate spam submissions.