> On desktops it mostly doesn't matter because you can shift the keyboard over relative to the screen.
It does matter. The arrow key cluster and numpad take a lot of space, which means the left side of the keyboard (where asdf / your left hand are) and your mouse have to be very far from each other. This leads to either putting the keyboard's letter key section in the center, making your mouse be uncomfortably far to the right, or having your mouse in a natural position for your right hand, leaving your left hand way out to the left. Over time this can (and did, to me) cause posture / muscle balance issues.
Currently I'm using the Microsoft Sculpt Wireless keyboard. https://www.microsoft.com/en-ww/accessories/products/keyboar.... The main keyboard with letters and two extra columns just to the right of the letters (with no gap) means everything can be positioned perfectly. The wireless numpad is there for when you need to use it. It's a very overpriced product at $100 but works wonderfully.
Also, in college I had a tiny desk with a cabinet underneath its left side, so my chair was offset to 60/40% of the way to the right. With a full size desktop keyboard with numpad, there literally wasn't enough space for my mouse, unless I moved the keyboard very far to the left. Thus causing the issues I mentioned above. That's what got me into tiny mechanical keyboards :) I'll admit it's a somewhat niche case, but there are probably still many millions of people in the world with a similar desk setup to that for whom a full size desktop keyboard is an unhealthy option.