Low Domestic Spawn in Superflat Worlds/Passive mobs do not spawn on superflat outside of spawn chunks

When I create a superflat world, the spawn rate of cows/sheep/pigs/chickens seems to be incredibly low, whether it's run-of-the-mill superflat or a customized one. I see a group of animals roughly every 1-2km+. There tends to be some that spawn at world creation but it's essentially empty after that.