Why?

Well, just to get started, these three things are also “just” theories:

1. Germs are the cause of many diseases.
2. Everything you have ever touched is made of atoms.
3. The spinning earth doesn’t fling us into outer space because of gravity.

Would any reasonable person actually think the phrase “just a theory” makes sense for any of these three things? Use of this phrase, for evolution, the Big Bang, or anything else, indicates one thing: the person talking does not understand the meaning of the word “theory.” Theories are the best science has to offer, and science is the foundation of modern civilization. These theories are based on the repeated testing of hypotheses, using experiment, to explain what we observe — so they are evidence-based explanations, not mere guesses, as the annoying phrase “just a theory” implies.

Evolution is every bit as well-established a theory as the three examples cited above. All theories are subject to further testing, which is an important self-correcting mechanism in science. No theory is beyond revision or replacement, if new experimental evidence calls for it. However, that fact doesn’t make any particular theory invalid — it simply helps explain why science works. It also works just as well whether people believe in it, or approve of it, or agree with it — or not.

If you want to disprove the theory of evolution, just find a fossilized rabbit in a one-billion-year-old rock, as J.M.S. Haldane famously observed. It will only take one such finding to accomplish your goal, and you can publish your results, and become famous – if you can find such a fossil. For your own safety, though, please do not hold your breath while looking.