Honestly, not just ad networks. It’s also publishers. We tried 2 major non-google ad networks. The amount of scams and borderline scams were crazy. And apparently asking for some quality control is complicated. Even with google and ad-exchange, we had to raise the minimum costs by quite a bit to keep most of the scams out. This lowers revenue so most publishers have the same interest in fighting those scams as the networks.

The only reason to fight against the scams is because one cares a little about ones viewers (well, and I guess maybe a bit of brand safety). Which seems to not be the case for the vast majority.