Comment by alerighi

I think because it cost money and they get little benefit on doing so.

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.