I used to do this. Well I used to train them on upper body day and prioritize them on leg day too. Calves are/were a major weak point of mine so I wanted to be sure that they had a lot of attention. Also I find that if you don't prioritize them on leg day it is a real bitch to try and do standing calve raises with legs of jello from squatting. So to answer your question I imagine most people do them on an upper body day because there is no pre-exhaustion involved.
my calves need all the work they can get, so i do them separatly from quads
i actually try to give all my parts max attention, so i developed this split
shoulders/traps/tris
quads/abs
back/forearms
chest/calves
hams/bis
off
off