By the Constitution, The Senate can script the trial anyway they desire. It is their prerogative. If it is to be a fair trial, then it is only logical that President Trump should be allowed to profess his innocence and ask Majority Leader McConnell to call witnesses. Now it is President Trump's trial for abuse of power and contempt of Congress. It is not Joe Biden's or Hunter Biden's trial. However, this is all moot as it seems Majority Leader has decided to have a Potemkin trial with no witnesses.