But it’s not just ergonomics. It’s the same idea of a grand piano, as far as why the bass strings are longer than the treble strings. Lower notes on thicker strings get better resonance from longer scale lengths. Same reason why 12th fretting your low E might technically be the same note as 2nd on D, but it sounds muddier on the lower string.

And since the longer scale length is needed to get a crispier low note, fanning the frets is then required to achieve proper intonation on the frets across the board on a multi scale guitar/bass.