Because burden of proof is on the Plaintiff (civil cases) or the prosecutor (criminal cases). Defendant technically doesn't have to prove anything -- just throw enough doubt out there to plant a seed of doubt about the Plaintiff's or prosecutor's case.
Of course I do realize that.
I was just talking in the hypothetical.
But I read that Trump asked the same thing.
Obviously as frustrated as I was in the disgusting unfairness of it all.