I mean that's because sometimes the person I'm talking about isn't even on Mastodon. Sometimes they're on my Discord, sometimes they're on my phone, or in my messenger, sometimes they're on my TV, and sometimes they're inside a screenshot being shared with me from one of the various nazi websites I don't visit if I can help it.
More to the point tho, and please understand that this is not personal at all, I like most of the people who appear in my mentions; I don't care if you know. And I am actively incentivized to not tell you who I'm subtweeting because this is a public place and if you can see it, so can they; my life gets annoying and complicated after that - and since I don't actually care if anyone knows, including the person I'm talking about, it's just very low EV for me.
See pinned Tweet.