They still get a cut after all ...
I suppose the answer is the same answer to the same question asked to SE:
If RMTs/bots still pay subscription, if RMTs/bots keeps buying news accounts, why SE still takes measures against them?
The answer? That is up to debate.
I would say both SE and Youtube must consider, despite not hurting the revenue at first sight, in a long run if left unchecked the predatory behavior of those people exploiting the system will inevitably lead to the system downfall.
Youtube in particular has an extra argument. The platform itself is a hugh datalake that can be used to train new models.
But if most content on Youtube doest have human value embued, then the models arent learning new things, thus, it does make sense Youtube to want content that is at least some bit enriched by human insights, than straight AI generated content.