I cant see a physics reason why the fretboard on virtually all stringed instruments (except possibly a classical guitar?) are narrower at the nut.
Practically you need just as much room up there for finger access as you do lower down.
. I think a straight beam might put the relief higher up the fret board which would be less useful.
I doubt it's due to relief. I go along with the earlier answer of the ever closer fingering as one advances up the neck. I can't think of one western instrument, bowed or plucked that doesn't have a tapered fingerboard. Lutes are tapered, baroque citterns, viols, renaissance guitars. All the early instruments that have been through my hands such that I can't think of one single example that has not been tapered. Maybe something like the Chinese Erhu but that has only two strings.