It's more nuanced than that, and "in Dark Souls, every monster is inherently hostile toward you" is not true.
Most games have a clear division between hostile mobs you kill for XP and loot, and story NPCs which you cannot / are not supposed to attack.
That line doesn't really exist in Dark Souls. Most (all?) story NPCs can be killed, which has specific consequences if the player chooses to do so. And there are monsters throughout the game world that are functionally identical to hostile monsters - they look the same, drop the same resources if you attack and kill them - but are simply not hostile to you and are just minding their own business.
It IS more subtle than in other games, and might not even be obvious to the player at first. This gradual realization was actually one of my favorite parts of playing Dark Souls.
But there are definitely intentional gameplay elements that support this, it is not strictly text lore.
It's even more complex than that, because a small number of NPCs, if not attacked and killed without provocation, will go on to kill certain other NPCs.