Because upvotes also have the effect of elevating the visibility of content. Therefore, very very often, I upvote content specifically to promote it whether or not I “like” it. Sometimes because it feels important, sometimes to support and encourage someone brave enough to share something they created or love even if it doesn’t actually interest me at all.

Having my upvotes exposed to the public doesn’t give me an opportunity to explain why I chose to upvote that specific thing.