I'm not an expert so don't quote me but my guess is because the more limbs laying about means the more the items gpu has to draw, for any good pc that probably won't be an issue but for consoles that could become a problem. As kyle says there is a mod for this so if you're on pc feel free to download it.
Because this game was not going for that. Most games with dismemberment and gore focus on it. Partially because it’s distracting from other features and partially because it’s overly draining on the PC resources.