>And then the defense catches on
And has to redo everything. It's whack-a-mole.
Doesn't have to be powerful actors really, either. There are many less powerful actors who would love to get selected things removed from YouTube.
This isn't the same as spam mail filters, because you generally don't know what sort of emails other people are sending each other. If you did, you could deliberately try to manipulate filters to flag your competitors' emails as spam.