Stabs are the most common knife attacks. A stab under the front ribs matches the relatively common (so far as such can be said of knife attacks) attack of thrusting forward and up from relatively close range. From the front, that goes under and perhaps to the heart. From the side, the same height attack has to deal with ribs.That's true if the knife attack is the classic SD drill straight forward stabby stabby type of thing.
I don't know how often that's a real situation, but I'd guess it's low numbers...
I know that's not how I'd handle a knife.