Because of exactly situations like this! Say you're an advertiser and you make an image banner. You make a deal with an ad network to show your ad on some pages. Some site makes a deal with the same ad network to show ads on its pages. Your ad is on the publisher page, users are seeing it, everything is good.

Now a scummy publisher signs up with this ad network, and sets the ads to display underneath something else. If the ads are just simple images and nothing more, there isn't a way for the advertiser or the ad network to tell that they're being scammed.

(Disclosure: I work at Google on making ads declarative so they don't get to do this sort of thing. Speaking for myself and not the company.)