Which means that if the ad companies have consequences for letting through scams, new identities will face restrictions (e.g. identity verification, initial safety deposit, not being able to advertise on most surfaces) until they find a way to keep the scams out or die trying.

Of course, this will cost money, both because it will cost them legitimate customers that get caught up in the friction and because implementing the measures/reviews has costs, so they will only do it if forced. If they can just externalize the cost (in the form of scam victims getting hurt by scams) and/or can profit from the scams without sufficient downsides, they're going to do just that...