Why do custom menu shortcuts require the addition of the ctrl key?

Hi so this requires a little explanation:

I draw on a laptop that folds over the keyboard so I don't have access to any keyboard shortcuts, to remedy this I have a custom programmed numpad that I use instead. My college class is having me paint with Photoshop so I'm setting up my shortcuts to match my numpad and when I tried to assign the "undo" command to the letter "a" on my numpad it told me I have to use the ctrl key in my shortcut.

Is there any way to bypass this? This is the first time I've ever encountered a drawing program that forces me to use a combination of keys for a shortcut and I have no idea why they would put this limiting factor in place?? I tried searching around on the internet but couldn't find anyone else asking this question so I'm hoping somebody here could give me their thoughts on this.

I am incredibly frustrated by this pointless limitation

(Attached is a screenshot of the error when I try to make the shortcut for undo just "a")