The burden of proof is also a burden of time, literacy, and access.

Proving something takes time you may not have, language you may not use fluently, and access to records held by other people. All of that sits on the person making the claim, on top of whatever they are already dealing with. So the requirement to prove is never only about evidence, it is a filter for resources, and it filters hardest against the people with fewest.

If you have to prove something, start collecting the paper before you need it and ask what specific documents would settle it. If you set the standard, ask whether the evidence you require is realistically obtainable by the person you are asking. A fair test that nobody can sit is not a fair test.