When we don't have a big sponsor for a section of our site, or haven't had enough direct sales for certain placements, it goes out to the ad exchange to action the spot off. That's where the problem is. That's somewhere we could start enforcing certain quality.
Ad exchanges and real-time bidding (RTB) are only causing a problem for people working in the media space because on the one hand if you don't have them you lose money, but on the other hand it's like gambling in regards to the quality of ads that will be served.
If I was buying a box of cereal, "we couldn't source enough reputable corn, so we topped it with melamine" would be the death of that cereal, for me.
(And yes, this is buyers at the cereal company instead of ad sellers, but it's the same: the hypothetical cereal company was not able to "sell" enough of its money to corn suppliers.)
Of course the goal is to operate at 100% efficiency, but in reality that's not going to happen all the time. Backfill channels like this are a way to make at least a little bit of money when sales a slow for a bit for whatever reason.
In terms of the problems the online media industry faces with advertising, the efficiency of in-house sales teams is not a huge priority atm.
I'd just like to add that almost all the problems with malicious ads are caused by ad networks that are used as "backfills" for when the in-house ads aren't booked. Backfilled ads are not inspected and can contain everything, but they are also needed because otherwise you'll have unsold inventory.