I think the main problem is that this type of reasoning can't actually be proven scientifically, even if we have a study there's not a guarantee it's unbiased (who do you think funds research on advertising effectiveness). Then there is the problem that every product or brand in modern advertising is likely one of the handful of pseudo monopoly brands. One might argue that a person bought their product because they heard it in an ad, but in reality they might not have really had much choice, that makes it hard to say if people buy the products because they're familiar or if they just don't have much option.

The main point I'd like to make is that advertisers would like to believe they aren't wasting money or time, they need people to believe it in some capacity, because if enough people don't, eventually the dumb and blind companies who give them money will realize it too and stop giving them money. That's why the ad-funded internet is considered a bubble, it's not worth it, or necessary in a lot of cases, and the moment the dumb and blind corpos realize that, they'll stop dumping money into a hole.