Data doesn't have to have an exact fungible value for it to be a good way to divide up costs. Users torrenting terabytes a month should pay more than someone who mostly uses email and reads news articles.
Also, note that Netflix gets its CDN boxes hosted for free. It "lets" ISPs host them, rather than paying for colocation space. Charging equally for data use regardless of CDNs or not would rebalance this effect.
And finally, the largest cost for building out residential ISP networks is last mile. It's the most expensive part to build, maintain, and upgrade to support increasing usage. The CDN boxes are on the far end of this, so mostly irrelevant anyways to this point.
Also, note that Netflix gets its CDN boxes hosted for free. It "lets" ISPs host them, rather than paying for colocation space. Charging equally for data use regardless of CDNs or not would rebalance this effect.
And finally, the largest cost for building out residential ISP networks is last mile. It's the most expensive part to build, maintain, and upgrade to support increasing usage. The CDN boxes are on the far end of this, so mostly irrelevant anyways to this point.