Simplicity is a result of equilibrium, and seeking simplicity is a human specific preference
Simplicity is that a relative few theories and mathematical models can explain a number of phenomena. While complexity is the opposite where there seems to be an unending need to invent new theories. By this definition, physics and astronomy are in the former camp and social science and biology belong to the latter.
Why is the universe is even understandable? This itself is hard to understand according to Einstein. I propose a line of reasoning here. Simplicity is a result of long term evolution in a close system. The resulting equilibrium gives rise to simplicity. The infinite possibilities of any member of the system have been largely reduced to a highly confined options. Most of the possibilities are prohibited due to forces that have long been cancelled out during the long evolution. Because of this simplicity, there appears to be causal effect. In other words, causal effect is a direct product of simplicity. Take our universe as an example, the universe is in equilibrium by and large. Only a handful forces remain. Because there are relatively few forces and laws, the universe appears to be orderly and thereby allows mathematics to even exist and work. Mathematics owes its existence to the equilibrium of the universe. Equilibrium brings orderliness and slowness to change. Just imagine, if one puts one stone by another stone, and because the stones decay so fast, by the end of this action of moving them together, one counts zero stone. The law of addition will be forever different from what we know today. In this sense, math and physics have 'this worldliness' feature, and is a localized knowledge to this universe at this phase of equilibrium. It could be vastly different in other possible states of the universe or other universes.
One notable exception to the simplicity in universe is the complexity in bio-sphere. Because the bio-sphere is inherently expansive and interactive, we cannot reduce the theories to a few laws and mathematics models. The bio-sphere is NOT an equilibrium system. Therefore it is very hard to apply causal effect to explain human society for instance. It is very hard to generalize theories or apply mathematics in bio-sphere or human society, as we are able to in cosmology.
Humans' brain is wired to understand simple things and not complex things. We seek patterns and generalize. This skill helps tremendously in our evolutionary past. For instance, our eyes are adept in figuring out linear movement. Our eyes are especially good at spotting moving object in a static background. The predisposition to seek simplicity gave humans survival advantage in its evolutionary history. We appreciate simplicity over complexity. Humans process limited computational power. It is most efficient to apply the limited resource to a fast algorithm. The design principal of the fast algorithm is simplicity. There is an aesthetic side of simplicity, whether it is a new physics theory or a design of a gadget. The propensity of seeking simplicity is a very human specific trait, and has nothing to do with the reality whether the world is simple or complex.
The coincidence of the simplicity of the universe and human's preference of simplicity is fortunate and fruitful. Specifically in the math and physics the coincidence yielded amazing results. There is no reason to doubt that more amazing discoveries will surface in the future. However, a grain of salt must be added so that we are conscious that there is less mysterious processes or agent involved in the coincidence. This article hopefully explained why.
