The idea is that he could have, but it was through his suffering that mankind would be saved. I’m not a believer but from a literary perspective, the story is that if he saved himself, all humanity would suffer. His human nature wanted to survive and avoid suffering but to do so would go against God’s will and humanity would not be absolved of sin. He overcame his human nature to embrace the will of God. Hence his prayer in the Garden of Gethsemane, “My Father, if it is possible, may this cup be taken from me. Yet not as I will, but as you will”

By the way, the nails actually go through the wrists in crucifixion, between the bones (radius and ulna). If you put the nails through the palms like they so often show, the weight of the body would just make them rip through the meat of the hands between the fingers.