I thought like that too until I realized something. Recently my cat started having some symptoms that weren't going away. The vet treated him for an upper respiratory infection, then for an allergy which isn't too common in cat, but nothing was working, finally he was diagnosed with a nasal tumor, VERY rare in cats! My point is that when you (or your pet) go in with symptoms they are going to treat the most common thing. I mean if I took my cat in and all of a sudden the vet said he's going to anesthetize him and X-ray his sinuses and take biopsies I would have given him a sidelong glance as I grabbed the cat and ran out, holding onto my wallet.
So that's why, most of the time it IS a horse... and going right for the rare problem just wouldn't be prudent for ANYone.