Even if it would be easy only pay per conversion, you would create a different type of perverse incentive in which ad platforms would exclusively want to target

Even if it would be easy only pay per conversion, you would create a different type of perverse incentive in which ad platforms would exclusively want to target consumers their models attach a high preexisting (before exposure to ads) probability of conversion. That is OK for lower funnel tactics but it would severely reduce your ability to put your brand in front of a new audience.