Because it's science. An easier example for you might to understand that a virus causes the common cold. You might say Darkstorm is the cause of the common cold and science will say it's a virus. But they will say 1) we are sure that there are microscopic organisms that cause a variety of sickness so there is precedence. 2) we can find a particular virus in the blood of people who have a common cold. 3) They may also test this by exposing some people to this virus and others to a placebo.
Gravity has it's own lines of evidence starting with the establishment of the reality and testability of spacetime with special relativity.
Going with the geometrical interpretation, general relativity then defines gravity as the curvature of spacetime due to mass. This theory expands on the Newtonian equations so we can make many new predictions using the theory.
First we can use it to understand the strange behavior of Mercury's orbit. Then it predicts gravitational lensing, which we observe, it predicts the universe is expanding, gravity waves and like special relativity shows time dilation in a gravity well and exactly how much depending on the mass/distance of the object.
With GPS satellites we find the error correction is exactly what relativity predicted.
Our current physics are why you are sitting at a computer or cell phone.