Or because it’s stupid political theater giving Trump’s base an out when he inevitably says some dumb bullshit during the debate and loses. He obviously never expected that ridiculous request to be granted. The act of playing along is an act of weakness, so it’s never going to happen.
Also they’re both going to be on drugs. All presidents are given various drugs. Some to keep them alive, some to help them concentrate. It’s no big deal, it’s a demanding job.
Edit: forgot Obama actually did show his birth cert. Shouldn’t have played along though