Also because a bow with the right tip focusses all of it's power into a MUCH smaller area, than a bullet does - small enough that for woven / fiber based materials, the top my actually pass BETWEEN strands, letting the rest of the arrow-head part the material, instead of having to actually break it. (Plus, the sides may actually be blades, further reducing the efficacy of stuff like kevlar).

For the matter of things like hydraulic rams or automatic winches, those might be viable options, just with their own drawbacks.

They'd certainly be noisy, for one thing. They'd also be exerting tons of force on the frame of the crossbow, so they'd be restricted to the bigger sizes, or maybe even require special reinforcing. And they'd also be bulky as hell, adding to the Concealability of an item. So yeah, if you want your troll to walk around with a giant hydraulic crossbow that shoots rebar through schools, good luck hiding it.

As for things like FA fire? No way, no how. Not feasible. At the very best, SA fire. Reloading crossbows is slow. It always has been, always will be. If you want something that reloads faster, use a gun.

~Umi