how you break a single player game?
You make it no longer enjoyable to play because you become overpowered and godlike. Or you level up "incorrectly" and are severely underpowered.
why has spell creation been taken out.
I'm not sure anyone knows for sure. Lore-wise, I would hazard a guess that because magic is not seen as a good thing in the region.
why are enchantments on armor piece specific.
I suspect to keep some semblance of balance.
how does a personal play style, make something "OP"
When you become a god and you do not want to be.
and why does "self gimping" make so many people upset.
Because the people who refuse to self-gimp claim that the game should gimp you, you shouldn't have to gimp yourself. I don't understand it either.