Back in the olden days where ads where more sparse, they would have so much more impact. Nowadays after the awash of ads we have everywhere, companies are collectively loosing their ability to make ads impact. (by logic, the more we are exposed to it, the less effective it it). Therefore they did it to themselves. They abused it, so that now people reject them where they can, and it becomes less profitable. No sympat…
The big brands have figured out that in cost-per-click (or any cost-per-action model) retargeting delivers the best results, so if you go and search for flights to Barcelona or some shoes, those flights and those shoes will follow you around - there's a high probability that a click will lead to a purchase, and it costs advertisers essentially zero if they're wrong, since you likely won't click.
The Web ads industry thus turned into a raffle industry - ads for same flights to Barcelona will turn up on outdoor enthusiast sites, CNN, Wired magazine, ESPN, your local newspaper site, Facebook and whatever else you happen to visit the next few days. In a raffle scenario the entity with the most raffle tickets gets higher chances of winning, so publishers are locked in a war to accummulate as many raffle tickets as possible with the hopes that one of them will be a major winner, i.e. you finally buying those tickets to Barcelona and those shoes on Zappos.