They are still custom because people aren't all exactly the same shape and size even before amputations. That said though I presume you don't want small pieces of bones or like partial smaller bones beneath a prosthesis. Like it makes sense to keep fingers on a hand. But if it's just an arm then wrist bones that doesn't seem ideal. I'm not a doctor. But it seems to me if you have a choice how a limb is severed there's probably a "best" way to do it.