I think that fretless basses have long, straight featureless necks/fingerboards without much point of reference. The violin family has a few points of reference, like the stop of the neck and body contours that help key in position memory. Those instruments also are tuned in fifths, so there is a little less need to shift up and down the neck (to some degree, and depending on the music being played). I can't comment on upright basses since I don't play them, but even they have a stop of the neck that can be used for reference. I used to chase the cool factor of having an unlined fingerboard, but like others posting here, I also found that intonation only improves when there is a lined fingerboard to complement the ear.