The issue is that large-scale advertisers don't actually trust the ad networks - the network could make up data, and has less reason to track fraudulent clicks/etc.
Yesss, and with video ads, where VAST is involved, once the ad script starts loading, it has no knowledge about the actual media file, as it has to follow a chain of XML files leading to different middlemen (for tracking purposes), ad exchanges, etc... Bidders in ad exchanges can be exchanges as well.
Dealing with bots, fraudulent clicks and malware is one of the things that sets some ad platforms from the really nasty ones.
Dealing with bots, fraudulent clicks and malware is one of the things that sets some ad platforms from the really nasty ones.