Because they are not "up-to-date weapons"
The average civilian would not consider owning/purchasing a WW2 weapon if any weapon at all, they would want something cheap and reliable that they could use for self defense
The crazy civilian would just make up they own weapons out of a pile of stuff, and anything could be in that pile, including a WW2 weapon. So, if we were to add any WW2 weapon, it would have to be modified in a way that, say, a crazy semi-genius would invent