Is it because it would break compatibility with some devices? Is it too expensive?
(not that I'd like that; I always download videos from YouTube for my personal archive, and I only use 3rd party or modified clients)
Major platform like Netflix etc. don't implement that DRM since they care, it's because they content they distribute requires that they employ that measures, otherwise who produces the content doesn't give it to them. Content on YouTube does not have this requirement.
Also: implementing a strict DRM on all videos is probably bad for their reputation. That would restrict the devices that are able to play YouTube, and probably move a lot of content creators on other platforms that does not implement these requirements.