So what’s “short” about shortcake, or short bread, or shortening for that matter? A short cake is “short” because it doesn’t rise very high. That sounds like a glib answer to that question, but it’s factually true.
Fat, by gunking up the bonding sites at the end of gluten molecules, prevents strong gluten networks from forming. This has the effect of making baked goods tender. But since strong gluten networks are what trap steam bubbles enabling bread to rise, fat, a.k.a. “shortening”, has the added effect of keeping whatever it’s added to rather flat.
You’ll sometimes read elaborate explanations in the food press about short crusts being “short” because the gluten strands they contain are “short”, or some such thing. But in this case I think the simplest answer is in fact the right one. Short cakes are short because the are lacking in the area of being high. Bereft of verticality. Possessing an absence of tallness. I think that’s it.