I think it is because mobs need a player around to spawn.
There might be chunkloaders with fakeplayer but i'm not sure.
Pretty much, I believe mobs will only spawn in player/fakeplayer-loaded chunks, there used to be a distance-from-player limiter that defines a "safe" area around the player that mobs could not directly spawn in.
Also, I'm pretty sure that there are a few chunkloader mods that use fakeplayer, but last I heard, there was a bug with it that either rendered it useless or caused a crash. The Forge devs have probably fixed it since, but do be careful.