CSAM is highly illegal, no doubt these accounts are getting banned but likely not charged (who knows what jurisdiction the advertiser is even in) clearly they're able to pass the current bar.
This is conjecture but I'd bet that in all likelihood someone at some meeting at Meta has floated the idea of moving impedance to the signup to try to limit this phenomena. Maybe more verification, or more hoops. And likely Meta crunched the numbers and was looking at billions of dollars lost in legit ad traffic and that just doesn't fly for a publicly traded company.
I'd guess that the folks at Meta would absolutely love CSAM to stop showing up, It's simply too dangerous, likely costs more than it earns, makes them look bad when it shows up, and alienates users who don't want to see it.
But those negative effects are probably all quantifiable to some dollar value that's less than the alternatives.
Executives don't get bonuses for altruism or ethical behavior. They get bonuses for making more money than they did last quarter. real fix is changing the game but that's a lot harder than it seems and can have many unintended consequences of it's own.