Because the block button doesn't actually block. It poorly hides the user's content from your view while other users can still see the abuse.
Why would that matter? What kind of useable communications platform prevents third parties from conversing about a separate person?
How is that harmful to the original poster? They don't see it.
Seems adequate, especially when defining a universally accepted definition of "abuse" that isn't just censorship-as-abuse itself is such a hard problem..