Also like how they avoid making him overpowered. Instead he's just impossible to kill, and he can even have children like everybody else. The problem is that they're normal, and he always outlives them. Which is probably part of the curse. He will see his legacy die before he does.
It does fit the fact that Caine is supposed to be blessed with eternal life and a mark born to show who he is and killing him... is a terrible idea.