It's a shame that, after 2 expansions (with allegedly no more ever to be made), there is only 1 card with the animal symbol on it. What manner of dairy injustice is this?!
I'm only guessing, but I would say that because the Cheese Factory is activated on a 7, and that's the most common number to be rolled on two dice (that is, until you need to roll it in a game of Catan, when it suddenly disappears from the dice), any more buildings that provide animals would make the tile considerably over-powered.
I think it's nicely balanced as it is (in the base game, at least) due to players not starting with Ranches, and having to purchase them initially. It does lose some of it's power when mixed with expansions though, as you really need a couple of Ranches to really notice it's benefit, and there's no guarantee they'll come out at all.
EDIT: I must watch too much Big Bang Theory, as I always thought it was the Cheesecake Factory until I just checked!