They already had the proper-DRM tech for youtube movies for years, why didn't they already turn that on for all content?

It would break many millions of old consumer devices that no longer receive updates, like old smart TVs. They are waiting for that old device traffic to drop low enough before they can force more robust measures.

You already need such things for certain formats.

They actually somewhat did: https://github.com/yt-dlp/yt-dlp/issues/12563 "DRM on ALL videos with tv (TVHTML5) client"

It's not really a matter of just turning it on when it comes to the kind of scale that YouTube has on their catalogue. It's practically impossible to retranscode the whole catalogue, so you're more or less stuck with only doing it for newly ingested content, and even there the tradeoffs are quite large when it comes to actually having DRM.

I think we can safely assume that the only content under DRM at YouTube today is the content where it's absolutely legally necessary.

YouTube's delivery scale is enormous and adding additional complexity if they don't have to is probably considered a no no.

But if they decide they have to, they can do it fairly trivially.

YT probably HAD to put the DRM on in order to get the license deal with the studios. Nobody is twisting their arm as much so other interests (wider audience, less server side resources, not getting around to it) can prevail.

They have to pay a technology license fee per month per protected title.