My theory is that some people see use of a mouse as a sign of noobery - which is true in some cases! Who do you think is more pro - the person who clicks Edit->Copy, or the person who clicks Ctrl-C?
So their lizard brain thinks "mouse=stupid, keyboard=smart" and then they optimise for a sense of superiority.
I prefer the keyboard due to severe tendonitis in my wrist and arthritis in my hands. I can use a keyboard all day with no problem. A few hours with a mouse leaves me all swollen and in pain.
I used to get what I thought was RSI. I mean it was RSI. I tried all sorts of weird keyboards and mice (don't bother with vertical mice, they suck).
What it turned out to be in the end was a poor desk setup. My desk wasn't deep enough so I wasn't resting my forearms on it very well, and I had a crap chair.
With a deep desk and a fancy mesh chair (HM Mira; expensive but worth it), I haven't had any issues since.
Maybe that's not the case for you but it's just a suggestion. Definitely made me suss of all the ergonomic keyboards and mouse - they made zero difference, and a better chair and desk completely solved the issue.
I wonder why you'd imagine that ergonomic input devices would solve a problem with your seating arrangement? I think generally they solve issues that are left after posture has been fixed.
Switching back and forth between keyboard and mouse is a pain. I used QB a lot at my last job but current one has too many things which need passwords entered frequently so I am reduced to vimium and chrome.
So their lizard brain thinks "mouse=stupid, keyboard=smart" and then they optimise for a sense of superiority.