I'm not familiar with Dragonball Z (I know if it, but the style is really not my cup of tea), but I am familiar with Mortal Kombat. My explanation for that is either that they aren't really humans despite appearances, or there is supernatural training not being called out as such (messy incoherent worldbuilding in my opinion, but to each their own). In the case of Mortal Kombat, this is understandable for a fighting game that doesn't really care all that much about its own lore, and certainly less than they care about gameplay. I feel quite the opposite.