Upvotes are a cheap. You can breeze through the site upvoting all sorts of things.
Really useful questions get favorited. Why not have the reputation system reflect this?
There's a disincentive for people to favorite things that aren't that useful because it degrades the utility of the favoriting system for them so you could probably award more rep than for a standard upvote.
On an unrelated note, the FAQ does not accurately reflect how rep is earned as it does not mention getting rep for accepted answers.