Because it wasn't built that way. You have not merely put the cart before the horse, you have put the woodcutter before the...whatever the midwife-equivalent is for a veterinarian.
In our world, we stumbled upon existing symmetries, and from those things, learned more about what the world is--and, importantly, what it isn't. Indeed, symmetries are one of the most fundamental aspects of existence, as proved by the criminally forgotten mathematical physicist Emmy Noether. (And yes, this is a proof, an objectively true mathematical fact, not just a durably-observed pattern!) In layman's terms, Noether's Theorem proves that, for all systems that can be described by a certain extremely basic mathematical structure, if that system exhibits a symmetry, then it necessarily has a conservation law. As an example, the fact that physics worked the same five minutes ago as it does now requires that energy is conserved; the fact that physics works the same when rotating to the left vs to the right requires that angular momentum is conserved; etc.
The Great Wheel was not developed by uncovering symmetries. It projected them, enforced them, regardless of the consequences that might be entailed. That is a vastly different situation. There is no possibility of learning what the world isn't--or, indeed, even learning what it is!--from the symmetries of the Great Wheel. The Great Wheel's symmetries were declared to be true, and now we have to live with whatever contradictions or baggage that entails.
IRL symmetries are us, the denizens, discovering what is just observably true about our world. When we engage in fiction-writing to create a cosmology, we are not denizens of that cosmology observing it. We are the gods themselves--or, I guess, in Great Wheel terms, the over-over-gods, above even Ao--willing that cosmology into (fictional) being. It is not empirical in the least; it is creationist, as is all fiction from the author's and reader's perspective.
