Is this really true though? Don't they have ways of tracking the returns on advertising investment? I would have thought that after a certain amount of time these ad buys would show themselves as worthless if they actually were.

No, it's not really true. Media companies have a whole host of KPI's and tracking methods to evaluate the success/failure of their campaigns/single ads: here's a small summary of some of the KPI's and methods https://www.themediaant.com/blog/methods-of-measuring-advert...