Two reasons: 1 (weak reason) -historical reason. Over thousands years we always had gaps in knowledge which we attributed to gods (spirits, etc) and which later were filled with knowledge which explains phenomena WITHOUT need of gods or spitits. Now there is literally no gaps left big enough to put any small god or demon in. So, history shows that what was demon/god duty yesterday, tomorrow becomes natural (=mathematical) event.
2 (strong reason). All known natural laws (which govern all events) are simply results of symmetries around us. Energy conservation law is symmetry of time - the fact that nothing in universe depends on time. Whether you run any experiment today, yesterday or tomorrow, there is no difference. That is what is called invariance (=unchange, or independence) of all events on position in "time axis". Time is, so te speak, not physical entity - nothing depends on it).
Same with space - nothing depends on position in it. Whether you run any experiment in point A or in point B is irrelevant. This symmetry (invariance on location in space) results in momentum conservation.
Independence upon direction in space (run it east, west, north, south, up, down in space - no difference) results in angular momentum conservation law.
There are some other symmetries wich result in existence of antimatter, in relativity, and in all forces we know.
And those natural laws (=mathematical symmetries) rule each and every event in universe. So, there is no room for gods - all events obeys natural laws. So, mathematical symmetries RULE and SHAPE all in universe.