Intrusive ads are more profitable for the ad company, while the costs are largely born by other parties. A strategy to privatize the gains and socialize the costs is common in a lot of sleazy industries.
Yes, it is hard to scale Terms of Service enforcement. Yes it is a hard problem to solve finding bad actors at scale. That shouldn't be a free pass to just not do it at all. Especially when money is changing hands. If someone is paying you to be a bad actor they are either paying you to look the other way (called a "bribe" in most jurisdictions, and illegal in some of them) or you aren't doing due diligence before accepting bad money (called things like "laundering" and "embezzlement" at scale). "It's hard to scale" doesn't sound like a good excuse to do financial crimes, last I checked with banking regulators and is in fact the opposite (a larger crime); why should Google or Meta get a free pass in advertising because they don't want to put the work in and take the revenue hit?
What evidence do you have that they are "not doing it at all"?
Yes, and so should every person that works for them.
"Do no evil. Instead enable others to do evil profitably and take a cut off the top."
That's true, but Section 230 of the Communications Decency Act of 1996 provides broader (but not unlimited) immunity for user submitted content.