It's because they're devoid of any actual content and uninteresting to every other reader. I agree they might feel nice to the OP in the case of 'Show HNs', but in those cases there's probably another way to reach out (and the higher effort will make it more meaningful).
(I could've sworn there was a point in the guidelines on this even using similar language - 'devoid', 'content' - but there doesn't seem to be now. Perhaps I've just read someone commenting something similar a few times!)
Something like:
I've bookmarked this for later review, good post (this partially exists as the upvote)
I like posts like this (an overload of the upvote)
I want to read what other users think of this, but don't necessarily have a comment of my own. HN, please comment (an overload of the upvote as well)
I think this post is off-topic (this one exists - its called the downvote for users with ~520+ points)
I think this post violates the guidelines (this one also exists - called flag)
# "Changed my mind" (icon: graduation cap)
In most communities, this is a overload of the upvote. But it's a particularly good kind of upvote, the complement of the usual echo-chamber "upvoting because I already agree". The icon+phrasing combo reframes "I lost the debate" into a positive thing, more like "I was educated". It's my far my favorite reaction to receive -- lots of warm fuzzies. In order to avoid confusion/misuse, we did not make this the primary upvote (that's thumbs up, with the verb "appreciate").
# Sympathies (icon: blue ribbon)
It's a bit tasteless to "like" or "heart" sad news (eg, someone passed away). At the same time, a downvote isn't appropriate, either. This fills the role of "support for the person posting, not the topic they're posting about".
[1]: You can see some of it, as well as the full set, here: https://community.snowdrift.coop/t/custom-emojis-esp-as-reto...
The only thing that's 'missing' is the public declaration; putting one's name to the upvote.