The reason for this is a cyclical phenomenon: When people download these kinds of videos, they often watch them when they are only 90-95% complete (whenever the final orgasm occurs), because the remaining 5-10% takes so long and they don't want to wait. But since they watch it and it is "used up" before the download is complete, they then delete the video. Therefore, the internet is full of transient peers that only have the incomplete file for a temporary time, making it almost impossible to download the entire video. This is comparable to waiting for a few moles of a radioactive substance with half-life of 3 days to fully decompose - which, theoretically, never happens.
"pass the fucking salt god dammit, my guild is raiding in like 10 mins"