You are using an incorrect definition of the word. Theory used in science[0] means:

a coherent group of tested general propositions, commonly regarded as correct, that can be used as principles of explanation and prediction for a class of phenomena

and not:

a proposed explanation whose status is still conjectural and subject to experimentation, in contrast to well-established propositions that are regarded as reporting matters of actual fact

We have a separate word for the latter: a hypothesis.

[0]:https://en.wikipedia.org/wiki/Scientific_theory

Because science isn't about generating indisputable facts. If that's what you're doing, what you're doing isn't science. The epitome of good science is a predictive and well-supported theory like evolution.

Adding the word "just" implies a misunderstanding of how the scientific method works, because it doesn't get any better than a good theory.

I feel kind of bad mentioning it since everyone's bound to pile on, but evolution is a readily observed fact (e.g. breeding of dogs), while it's natural selection that's the theory...