I suppose any bass guitarist could pick up an upright and quickly figure out where the notes are, but to play it with authority and conviction requires a lot of practice and a good teacher. While having similar function to a bass guitar, it is really a different instrument, and is very much more "physical".
I liked Ed's quote about your typical BG player having gone through their first 5 cover bands while the Double Bassist is still trying to project a decent tone across to the other side of the room! Or something like that!