Ad networks won't go for it outside of very specific and narrow use cases (high-end ecommerce) because they don't have control over the quality of the landing page, a crucial element in whether the user converts or not.
In case of facebook where their ad network controls the whole process (SSP + Exchange + DSP) they would only lose, besides the infrastructure cost, the opportunity to make money by serving another ad.
But for the majority of ad serving world it's not that simple. If you're for example a DSP and buy ad impressions from exchanges via real time bidding, you have to pay for the impression even if you can't charge the advertisers for it. So you lose money twice. You don't serve another ad which would make you money and you have to pay for an ad that won't be paid for by the advertiser.
Meh, crocodile tears. If they shouldn't have to pay in the rare case that my site is down, then I shouldn't have to pay in the more-common-than-not case that there is no real person behind the click. And note that I have an incentive to keep my site up, whereas they apparently have no incentive to keep click-fraud down, evidenced by the fact that it's more than 50% of the case.
