Because people mute others for a variety of reasons. It’s kind of the only tool in the toolbox in some ways.

So I don’t only mute actual spam from bots, I also mute bullies, accounts that post or repost nsfw without tags, and, on rare occasions, accounts that post good stuff but just way too much of it.

I’m sure that all of that except actual spam from bots is sought after by other people, and shouldn’t be counted as spam.

Proof of this: I regularly see people I follow in conversations with people I’ve muted.