Why does anyone care? Who here gives presentations? Do you prepare notes? Do you arrange to be able to see your notes as you present?
The president isn’t a one-man cowboy. He runs a vast organization and part of his job is hiring good advisors. Why do we want them to perform as if that weren’t true for debates?
Wait what’s the issue?
I agree when it comes to speeches. But for debates, I do expect the candidates to be able to show a basic understanding of major policy issues. It’s not like the questions are super obscure. If you can’t recall 5 minutes of executive level talking points on 10-20 key issues, you should not be taking the top office. Just like I expect our CEO to be able to jump in and talk about a vast array of topics that he/she has hired a great team to handle.
Yes I give presentations. I often prepare notes / talking points, and occasionally have those where I can see - though depending on the presentation format that is not always possible to have those in front of me when I present.
However where I never have notes is after the presentation when taking questions - I have to be able to answer intelligently to each of the questions sent my way. Now if I don’t have a good answer I can talk a bit about it and maneuver the answer to something I feel comfortable discussing and highlighting. Additionally, if I am in a meeting at work and someone asks me a question in an area I am supposed to be leading, I need to be able to speak to it even if I don’t know all the nitty-gritty details. If this is expected of me at my job, it should be expected of anyone who wishes to be President of the country.
If i were giving a presentation where it was impossible to say, “whoops, i misspoke, i meant x”, there’s absolutely no way I’d speak off the cuff. I would write out answers to every question l could imagine being asked.
Maybe that’s just me. 
Have you ever been part of a panel or roundtable discussion? The moderator asks some canned questions that you have prepared answers to, but you are typically sitting somewhere with no notes. Then sometimes there is 30 minutes or more of questions from the audience.
I don’t write out answers for expected questions on those - you just know your stuff and your talking points and you speak off the cuff. No notes to be able to reference. It’s not that hard with practice - and someone wanting to be President has had years of political practice answering questions to get to that point.
Now if our debates went into the minute details of policy you’d have a point. But they don’t. Even the more policy-oriented politicians like Warren when they debate still use broad strokes to describe policy.