It might be argued it's not worth the dev time and site resources to do this given it's only a comment upvote and especially as the comment disappears when the review is completed.
Possibly not much resources
When you cast your vote in the review queue, the site already checks if there is already a matching comment (it doesn't comment on your behalf if one already exists).
So while the scripts are there already doing this, instead of:
"if same comment already exists: do not add comment"
"if same comment already exists: do not add comment; upvote current comment"
Maybe it's still a fair bit of resources, with various DBs to be written too etc (?).
Worthwhile even though temporary comment
Sure the comment is not there long, sometimes it can be but to be fair "usually" it is not.
However, this is an opportunity to educate users, and most certainly new users.
Having comments placed on my behalf is fine, but it's given me a few reply comments from post authors.
Such as: "@James, why should this answer be a comment, you are wrong, I have an upvote" etc.
I'm not saying if my comment had an upvote or two it would resolve this, but it might ease it, and if nothing else it's a debate point:
"The site doesn't like answers like this, blah etc, and my comment has 2 upvotes so others do agree, it's not just me being obtuse/unfair".