I saw a post noting that the ZA protagonist does not show a reflection in a mirror. This suggests that the reason the player is unable to enter most buildings is that the player character is a vampire.
I believe every building (outside of shops/restaurants) we are invited to, so this would make a lot of sense.
iirc in an older game there was an NPC who complained about us barging in, maybe that was a programmer complaining about the meaningless interiors.
