evolution is essentially a tool (to explain science), hence, device. device need not be tangible, i.e. literary device. it is called the theory of evolution precisely because it explains a scientific fact.
scientific theory is used differently in science. it is a set of hypotheses which have been thoroughly tested and peer-reviewed within the scientific community. it ties together different facts to explain a scientific phenomenon, i.e. theory of evolution, germ theory. you might have been misled by your science teachers in the past. some educators are not well-versed in science and may have indicated that once a theory is proven, it becomes a law. it is not so. a law describes phenomenon, the theory explains it.
it is called the theory of evolution because it explains how speciation came to be.