You write code... using the numpad?
> There should be plenty of space in front of it for a full size keyboard.
That's not the reason. Most people are right handed and most keyboard have the numpad at the left. Most people also put their mouse at the right of their keyboard. This makes reaching for the mouse a major nuisance. Without a numpad or with a numpad on the side where you don't have your mouse, that problem disappears.
Now of course there are other pointing devices (trackpoint, trackpad, etc.) and some are located in the middle.
But the undisputable fact is this: most people are right handed and most people have a regular mouse and that mouse is located at the right of the keyboard.
Numpad sucks in that case.
Back in the days I didn't know about keyboards with numpads on the left of the keyboard and 60% keyboards weren't really a thing yet so although I'm right-handed I started using my mouse as a leftie (and still do to this day).
60% keyboard like the HHKB (one of the "OG" 60%) aren't about the lack of space in front of the monitor. It's about grabbing the mouse and coming back to the keyboard with less hand travel.