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.
Keep in mind that proficiency plays a big part here, too. It doesn't matter that a longbow uses a larger die if you don't know how to use it well. As for compound bows, they aren't separate like they were in 2E, which is sad, because it made bows viable for strength based characters.
Honestly the whole "dex is for bows" thing is dumb anyway. Real Longbows have ridiculous draw weights and are far more strength dependent than agility based. Dex is for crossbows and maybe shortbows would be fine enough, but Longbows should be completely strength based or at least have it be either/or like a finesse weapon.
