Nincada’s evolution is one of the weirdest and it’s entirely possible to evolve it and only get ninjask and not shedinja (full party, no regular pokeballs in bag, ect.) Shedinja can never be found in the wild (except raid battles.) So it follows that in the pokemon universe the conditions for a shedinja to be created are very rare. I believe that when a nincada evolves and certain conditions are met, its soul splits, with ninjask and shedinja each getting part. A semi popular theory is that random spirits possess the husk to make it a shedinja, but that wouldn’t make any sense as shedinja remembers all moves learned before evolution and keeps the same friendship and affection levels as before. This is the same pokemon. Maybe the lore about shedinja trying to suck the soul out of whoever looks at its back is just it trying to reunite both parts of its soul however it can. Thoughts?