Is there an actual reason you can't shoot the bow on its own?

[Nah]

I can't tell you how many times I've been incredibly annoyed by either just barely missing my input for the bow so I could fire it on the ground or the arrow grazing the head of a ground-based enemy because I wasn't exactly on the ground when it fired. Call this a skill issue all you want, but it really feels like it's not supposed to be as hard as it is to fire the bow when you want to shoot something on the ground. I get that this is a mobile game too, but why can't there be a bow button like there is a dash button in that case? You wouldn't even have to take out the input for air shots, just making it so you have the option would make it a lot less annoying.

[Jerry]

There is a Dark Divinity a bit later in the game, that lets you greatly reduce your bow shooting speed in exchange for more souls. As a side effect, ground shots get much easier again, compared to a fully upgraded bow.
I think, combining the jump and bow function is mostly because of various interface reasons, yes. As a button on its own, while practical in some situations, it would make the game more difficult to play on phones as well as computers. I kind of made my peace with the way it works now.