Here it is! Here is your precise misunderstanding. The fact that you cannot prove Intelligent Design is PRECISELY why it is not a scientific theory. A theory in science is a framework of knowledge that harmonizes an observable, testable body of empirical results. The key here is you must start with observations, testing, and consistent results. Once you have developed a body of results, you can harmonize those into a theory that explains the whole.
Take for example Germ Theory, which is the framework for understanding the operation of microscopic organisms that enter our bodies causing illness and infection. It is based on observations of the bodies immune system, microscopy, bacterial cultures. Those observations are tested with medical research, treatments, drugs and antigens. From those tests, we derived consistent results that have withstood rigorous scrutiny. After all of that, we were able to formulate Germ Theory.
Evolution Theory has taken an analogous path. We observed comparative biology, anatomy, genetic drift, embryology, and from those observations, we fashioned experiments on heredity, phenotype expression, population changes, and many more, resulting in repeatable, consistent results. From these scientific facts, we fashioned Evolution Theory to explain the mechanisms of observable scientific facts we had previously discovered.
The fact that you cannot observe, test, quantify, ID is exactly why it is not a theory. There are no observations of intelligent design. From those nonexistent observations, there are no tests of intelligent design. From those nonexistent tests of intelligent design, there are no consistent, repeatable results, or facts, of intelligent design. Without those facts, there can be no theory. As you can see, Intelligent Design is many, many, many, many steps away from constituting a theory. It is not slightly deficient, but iteratively deficient, having not the precursors, or even the precursors to the precursors, of a scientific theory.
As scientists, we should embrace alternate theories...
You do not get to use that word. You clearly do not understand what science is.