Why are users not allowed to view upvotes that other users have cast?
Many reasons, 1 as you stated:
a user upvoting someone simply because that user upvoted them
This reason on it's own is enough not to show them.
The site works well, vote as required based on question's and answer's own merits.
"Friendly" upvotes will spoil various things, such as the accuracy of a question or answer's value.
As well as users revenge downvoting because they didn't upvote - you answer a question, get an upvote or two, and see that the questioner didn't upvote you.
You revenge downvote them, they revenge remove your answer as accepted, etc, etc.
Even where people are savvy and don't do the above, there will be comments "hey questioner, why didn't you upvote me as I helped you..."
However unlikely you may think this and other issues are, think of millions of users all with different attitudes and morals and moods all seeing who's upvoted who....
Moderators cannot even see who has cast votes, only totals, which we see at 1k rep.