Both science and common sense long ago concluded that severed limbs don't regenerate in human beings. Science can account for this and may some day be able to overcome it.
However, if one believes in miracles, it's an absolute contradiction to rule out the regeneration of a severed limb. That would be no more a miracle than the cancer cures that are claimed all the time.
Praying for a limb to grow back is something a little more tangible; it's easier to pray for something that has a reasonable possibility of happening, by chance or by work, rather than something that the Christians know won't happen.
The fact that no one prays for a severed limb to regrow is proof that underneath all the posturing and pretense of faith, some rationality has survived. People don't even dare to pray for the regrowth of a severed limb because they know it's impossible.