There must be technical reasons that YouTube hasn't already used this approach, otherwise it seems that they would have done it by now. I'm guessing it would r

There must be technical reasons that YouTube hasn't already used this approach, otherwise it seems that they would have done it by now. I'm guessing it would require transcoding for every video played, and the energy cost of that (even with optimized GPU units for this job) just isn't worth what they get paid in ads.