The reason why we use the phrase "Theory of Evolution" is the same as why we use phrases like Theory of Gravity or Pythagoras' Theorem. The word "theory" in ordinary English and in scientific jargon has vastly differing definitions.
In English language a theory equates to a hypothesis in scientific jargon, basically an assumption predicting the end result of an experiment. If the prediction after numerous peer reviewed experimentation and data proves to be true or accurate to an acceptable level of probability, the hypothesis is accepted as a theory, doesn't mean it isn't true.
Scientific data and the results of scientific experiments are based on fluid, ever-changing variables, some of which are beyond our control hence nothing that we conclude from experiments are termed as established facts even though the chances of it not being a fact are less than 1%.