Because they're psychosomatic and many representations of Christ on the cross show the nails through the palms.
That wouldn't work, though, they'd soon rip right through flesh between the metacarpals, and the victim would start flailing blood everywhere (supposing they were properly tied to the cross) or drop to the ground. They'll last much longer through the wrists, lots of small well packed together bones, there.
Does the “gurantee” include prions..? 🤨
(If you can guarantee no prions — don't really care about the no one harmed bit, as long as I don't know them or they're on my shitlist —, and it's cooked in some way I enjoy — no fancy gourmet spherified vapour shit, thanks — then yeah, definitely, I'm no vegan or anywhere close, but I'd rather eat human than some other animal who can't consent or have done anything to deserve being murdered and eaten.)