[Mindwipe]

The main reason is that it hits their CDN cache efficiency a bit, which costs money, and it's another set of key management systems to look after and operate and at YouTube's scale you want to minimise them as much as possible.

It also paints a very big target on Widevine Level 3's back.

But ultimately it's just a financial equation. What Google are losing from ad blocking isn't quite worth pulling the WV lever yet, but given it has clearly become enough to take softer measures and the pressure they are likely under from music labels I expect a wider rollout will happen in the next few years.

If I were to predict it will probably initially be "any video containing label music or studio clips picked up by content ID", and maybe an opt in tag for other creators at first. They're the ones that are much more useful for monetisation anyway, and you don't lose all your CDN benefits at once.