It gets annoying if the answer to another comment is short.
Edit: My answerable question: Is there a documentation about why?
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Super short comments, in general, have little value.
From this meta answer by Jeff:
...perhaps one should consider populating their comment with something meaningful of 15 characters or more?
And, another meta question about this topic.
(That is the team's reasoning. I'm not saying I agree.)
If you really wanna submit a short comment, you can bypass the filter by adding a blank link like so:
Hi [ ](http://example.com) That will produce the comment 'Hi'.