I think you came really close to the salient point, but then missed it: Targeted advertising is, as you said, really not that much more effective as context-based advertising, unless you're an ad-tracking behemoth trying to convince companies they need your product.

The whole surveillance capitalism nonsense we live under is a way for ad networks to justify their existence. If you're a company that has data on everyone, you need to convince people they need that data to accurately sell ads, and targeted advertising exists solely for that purpose. The amount of cross-site tracking that these companies are able to do outscales what any company could manage to accomplish themselves, or even for a smaller competitor to step in, so as long as targeting users across the entire Internet is believed to be a must-have, these ad behemoths stay on top.