Because
1. It counts as engaging with the post which is not the intent ( I almost never give a negative reaction. If I don't like something I just ignore it )
2. There are clients that show the List of Likes/Reactions and these posts pop-up if Mute Lists are not used & pollute that page.

And like you yourself mentioned, what counts as a positive & negative reaction itself is a flexible contextual thing...eg.I use 😱 emoji, mostly as a faux-shock & never negatively.

So I am not sure it's useful to add reaction of any sort, especially if I can't undo it.
Would appreciate if you could reconsider.