This!

It's a game. When a merchant signs up to an ad platform (or when the platform is in need of volume), they are given good ROI, and the merchant also plays along and treats it as "marketing expenditure". Eventually, the ROI dries up i.e the marketing has saturated and the merchant starts counting it as a cost and passes it onto the customer. I don't know if this is actually done, but it's also trivial for an ad platform to force merchants to continue ads by making them feel it's important: when they reduce their ad volume, just boost the ROI and visibility for their competitors (a competitor can be detected purely by shared ad space no need to do any separate tagging). Heck, this is probably what whatever optimization algorithm they are running will end up suggesting as it's a local minima in feature space.

And yes, instead of banning ads, which would be too wide a legal net to be feasible, banning tracking is better. However, even this is complicated. For example, N websites can have legitimate uses for N browser features. But it turns out any M of the N features can be used to uniquely identify you. Oops. What can you even do about that, legally speaking? Don't say permissions most people I know just click allow on all of them.