[prophesi]

Yes, as OP mentions, machine learning can be applied quite well in other fields "like image processing or winning at strategy games."

And yes, they test out their new ML algorithms to see how effective they are. But they don't need the machine learning to create their list of "gullible people." Rather, they don't even -need- that list. If you just serve stupid, shady ads that look as close to porn as that platform can get away with, you'd get your desired click-through rate. The actual personalization aspect is just a myth to validate our invasion of privacy.

[doublekill]

They need that list for max profit. Just serving ads without an intelligent platform behind it to know where and when to serve them, will hurt the CTR (which hurts both the ad tech platform and the advertiser).

Why would ad tech companies gather data and invade your privacy, and then not use it to sell more profitable ads? That makes no economic sense, but is a costly form of voyeurism. The "myth" you refer to sounds like a poorly thought out conspiracy theory.