DRM is expensive. Very expensive in fact because it is basically non-trivial encryption.
A website with as much traffic as YouTube cannot afford to DRM every single video stream. There just isn't enough processing power and electricity available.
Netflix et al. have a tiny fraction of YouTube's traffic with more income per user due to subscriptions.
Plus YouTube's storage demands are many orders of magnitude larger. A maximum upper bound for Netflix is 1 PB I'd imagine. Archiveteam alone has selectively downloaded more than 3 PB. YouTube has, I'd imagine, a double digit exabyte amount of data stored + backups.