Single Key Bummer

Why are there so many controls that can only be assigned to a single key?

I like to configure a layout that lets me leave my left hand in one place and my right on the mouse. I use a lot of modifier shortcuts to do this, but in this game I am arbitrarily prevented from doing so. I’m finding it very frustrating.

It doesn’t help that the UI says “Bummer” each time I try and fail to assign a multikey shortcut to one of these controls that don’t support it. As if it is just some fact of life and not a deliberate and (seemingly) pointless restriction.

If there is some rationale, I’d love to hear it.