Because it's reasonable. You cannot disprove what has no evidence, and why should you look for it, other than normal single good science "college try" check? It's a great way to waste time.

If the concept is not obvious, the onus is even more on the person bringing it up. Otherwise we'd still have physics running on phlogiston, and medicine on leeches as everyone explored irrelevant ideas.

What you said is basically “the one who asserts must prove”, an old standard way to describe the burden of proof.