I think that's because these are two separate features. My guess is they considered connecting these two things and decided it wasn't worth the added complexity. Or another argument is that it is for bookmarking and that isn't the same as liking - maybe you want to bookmark it because it is so bad.
My question would be: how come users who favorite a question don't up-vote it? (assuming they have sufficient reputation to up-vote).
A third, not yet mentioned aspect on favorite, is the ability to find the question later to answer it. I often view SP.SE on iOS devices and see questions I would like to answer later on since handheld devices are hard to type on, or I don't have the time to answer right now.
To compensate for high no of favorites, there are badges available. I think the levels are 25 favorites for silver badge, and 100 for gold badge.
To overcome the problem, it might be possible to get reps if your question is favorited! 2 points for every fav, maybe?
Favorite feature can be used for different purposes.
One of it's best usage is, when you see a question which might interest you but you don't know it's answer but would like to know if someone else answer's it now or in future, so you favorite it.
Now this question can be a good question and deserves a up vote or it might be not be.
What I normally do is, I favorite questions that I would like to know answer of in future and if question is good I up vote it too.
E.g.
How to Programmatically change the text layout of a Sharepoint page?
I fav this question but didn't upvoted it as I wanted to know it's answer but I don't think it deserved "my up vote".
You can always view list of fav questions by going to your profile page.
How come marking a question as favorite doesn't automatically upvote the question?
It's not necessary that every question you want to know answer of is a good question and has to be up voted (e.g. badly typed questions).
