Google and Facebook have an incentive to use up advertisers' budgets. But they also have an incentive to keep advertisers happy by providing good returns so that they will keep spending / increase spending in the long run. Advertisers are not always perfectly rational, but I don't think they're as dumb as the article makes them out to be.
The simpler explanation for poor ad targeting is the bidding component. If it was the case that you were shown whichever ad had the highest predicted CTR, I'm pretty sure you'd be seeing some damn relevant ads. But instead you're seeing whichever ad has the highest (bid per click * ctr). Or worse, advertisers can also bid per impression, in which case there might be no relevancy component at all.
My guess is the car ads on YouTube mentioned in the article are that last case. Both of those components (YouTube and car companies) are notable for skewing towards "brand" advertising rather than "performance" advertising. They're not trying to get you to buy a car right then. They're trying to get you to feel good about their company in the long run so that five years from now you're more likely to buy their car.