We would have to come up with a reason to have crossbows rather than bows if this were the case.
This request seems mainly to be geared towards the fact that currently, bows and crossbows are practically identical and suck equally, and more specifically, aimed at giving modders far more freedom with what they can make, otherwise things like automatic crossbows, bolt-action rifles, slings, thrown weapons, and javelin guns are just a crossbow/bow with a different name and more often than not suck just as much.
Strictly without modding, the game has three ranged weapons. Bows, crossbows, and blowguns. Dwarves can only make crossbows and their ammo, unless the end user mods their game.
In this context, let's assume the end user doesn't plan to do so.The only sources for bows and arrows is trade and battlefield salvage/stripped POWs. This by definition means 99% of the time, most bows and arrows will be of very poor quality compared to the crossbows and bolts your own in-house producers can output, and supply is limited to whatever merchants or attackers bring.
- In the case of the former, arrows will often be of poor material, or hideously expensive if prepared meals aren't abused. I've seen more silver arrows than anyone should have to just playing as humans in my own game.
- In the latter's case, supply provided is erratic. An attack might be all archers or only have one archer and he fled off the map when the fight went south. Without cage trap abuse there's also the fact that a portion of the ammo will be expended by the enemy defending himself from your arrow-hungry militia.
- The best resource for arrows is elves, and obviously their arrows are only useful as training ammo, further diluting your ammo pool of what can actually be used to hurt things. In contrast, you can produce training bolts in vast quantities without antagonizing your trading partners from wood or bone.
They would also burn through usable ammo at a higher rate (say firing three arrows for every one bolt as a random example,) and not necessarily with any greater accuracy than a crossbowman. So they pay a price in rate of fire for needing to be cycled out to reload much more frequently on top.
So for someone who just plans to grab and go, crossbows are what they are given; they're easier to acquire, can be acquired at higher quality and in higher numbers, and so aren't limited to whatever you can buy or pry from the mangled corpses of your enemies.
Plus thematically dwarves are generally imagined at crossbowmen. There's certainly nothing stopping a dwarf from using a bow, but that's like giving a samurai an arming sword - they can probably make it work, but they'd be more effective with what they know and in a vanilla game, dwarves know crossbows.
Unrelated to all of the above, everything also fires at the same rate, which is absurd when bows and blowguns by their nature should be faster to fire than a crossbow. Right now, at least in my opinion, different ranged weapons are basically just flavor content since they're functionally identical in performance - that is to say, they're fuckin' useless unless you invest a ton of resources into training skilled ranged fighters regardless of the type of weapon they use or field them in such large numbers that you're basically playing a statistics game - doesn't matter if all your men are terrible shots if you have 100 of them shooting at as many targets coming at them down a narrow hallway.