[RockIslandLine]

Several people have given several reasons, many of which alone are enough to demonstrate awfulness.

The biggest reason they are bad is that they don't work. I would happily subscribe to a concierge service that showed me genuinely useful information. However, the advertising systems we have got are not built for my use case, they are built for someone else's which is adversarial to mine. I could not possibly care less about your quest to build brand loyalty and awareness.

As noted, people often get shown ads for products which they have already bought. When I buy a car, it's a safe bet that I don't need another car for several years. Every car advertisement that I see in that time is wasted effort for both the advertiser and myself. I also notice that nobody is advertising along the lines that might influence my choice. As a single, eco-conscious male, I don't need "three rows of seating" and I need "low zero to sixty" times even less. Automobile advertising is heavily devoted to the highly profitable SUV segment. Eliminate the Chicken Tax and promote decarbonized and efficient vehicles. Stop basing advertising campaigns around stoking even more antisocial behavior.

https://en.wikipedia.org/wiki/Chicken_tax

And the flip side, people getting shown advertisements for products they have rejected is also wasted effort. I don't need penis pills, and every advertisement I have ever seen for them has always been wasted effort.

Stop polluting my information streams with awful crap I don't need.

Nobody likes being watched. Instrumenting the interaction that I need so that you can make wrong inferences about my preferences is pretty galling. Undermining my control over what information I do give out is worse.

https://proprivacy.com/guides/super-cookies-flash-cookies

Further, not specific to targeted advertising is the method of advertising. Advertisements in popup windows are a terrible idea. Forcing me to take an action before giving me useful information is a terrible interaction model. And that's not designed to make me want to buy your product. Advertising when it exists should be as unobtrusive possible. Obnoxious behavior sucks.