Because you can’t prove a negative, the most common example is “there’s an invisible teapot orbiting between earth and mars”, there’s nothing you can show me that I can’t naturally explain as “you weren’t looking at the right place and the right time” or “it’s invisible, so obviously you can’t take a picture of it”, it’s literally impossible to prove it wrong.

The thing about science is that it’s falsifiable, i.e. it CAN be proven wrong. Everything science claims is easily proven wrong, whenever evidence comes that contradict current theories, a new theory needs to be created that also explains this newly observed phenomenon, and the fact that some theories have standard strong for decades or even centuries without any evidence been found against them is good evidence that they’re the best model we have for how things work.

And this is something most people don’t understand about science, if it’s not falsifiable it’s not science, you can’t make a non-falsifiable claim and call it science, which is why things like homeopathy or astrology are not science, their predictions either fail sometimes or cannot be tested. This means that every theory needs to show how it can be proven wrong. An example is that Einstein relativity theory says that gravity should affect light, so in his papers Einstein said that if we ever found a cluster of galaxies we should see a lense effect, because the light would be curved by the immense gravity of the region. Very recently we did found a cluster of galaxies, and we in fact observed a lens effect like Einstein predicted, had we not then that would mean there’s something wrong with his theory, but so far all predictions that we have been able to verify are correct which leads us to believe it’s the best explanation we have for that part of science.