Low, even very low, return of value is not no return. Therefore, given they make some return, and it has some value, that's enough for them to do it. Ads and ad data are two sides. We are often not the target for an ad, but our data provides stats about how an ad is performing. If more consumers are influenced to spend $1000 on something than not, then it's worth if for them. It's an aggregate cost benefit analysis not how effective it is at the isolated individual level.

Another thing to consider is that we should never fall into the trap of thinking we are immune from influence from advertisers. Firstly, it's basically what advertiser want; it allows more actions like this, more of our data to be sold and secondly because it's easier to influence someone if they think of a decision as their own choice, than if they think they were manipulated into it. We do not remember the ads we see but we can remember that we are all susceptible to influence.