A longbow is almost like a short bow, except it's a longer piece of wood and string so may be a bit more expensive. A composite bow would be special because it is a special design requiring some extra skill to make. In some cultures the longbow was the common bow to use, and I do think elves commonly use longbows, not shortbows. Thus it is natural to start out with a longbow if you are human sized. Halflings would have to use short bows, but humans or elves could easily start out with a longbow at level 1. The difference is 1d6 vs 1d8, hardly a big game changer. If you start out with a longsword it is 1d8 too.
Now why would you chose to use a short bow instead of a longbow then? Well if you're a goblin or a halfling you can't use a longbow because of your size.
