Shift Key Can't Be Keybinded

I'd tried to keybind Shift+___ when I first delved into the game. Is there a reason Shift+ is not keybindable that was discussed at some point in the past that I am not aware of as a relative newcomer to SotA?

I only mention this because I have keybinded Ctrl+___ instead as a necessary alternative for certain functions but have always found Shft+ super easy and intuitive for the fingers and Ctrl+ to be awkward and a strain to do quickly and accurately.

 

Let me ask this a different way. Is there any reason why the Shft key was coded to not be bindable as a Shft+___ function? I've never encountered this in a game before. Traditionally I've usually found reasons to keybind Shft+1-5/Q/E/R/F/C/V and the like. The Control key is clumsier as a substitute.