Because Youtube uses some measure of actually played video time to count a view. Depending on how it is implemented, downloads likly do not trigger that (since data loaded doesn't mean it was watched)
Hm, seems like that would leave a mechanism open for blocking those kinds of sites. One reason I can think not to is if other apps, like YT for Roku, have to access the data directly and then process the relevant view count/ad stuff client side
