Most webmasters hate CPA because it puts all the risk on them and none on the advertisers. If the advertisers doesn’t sell anything you make no money but gave the advertiser free exposure.
At the other end of the spectrum we have CPM (cost per 1000) where the advertiser has to pay a fix amount per 1000 ad views, whether or not the reader clicks the ad or buys anything. This puts all the risk on the advertiser and none on the publisher.
In the middle we have CPC (cost per click) where the advertiser pays for clicks received. This put risk at about 50/50 between advertisers and publishers.