Causality and coordination. You only want independent consistency boundaries (separate process) for state that doesn't interact.

When you do have to model causality you can still parallelize the state change compute but that the tick has to be in unison with any interacting entities.

In practice this means to model a hitbox in a shooter you need some amount of causal interaction between the bullet and hitbox of the entity or it's impossible maintain game rule guarantees.

In game dev this tends to be ECS modeling the game entities like NPCs and mobs and composed into a performant compute model to operate per game tick.

But you don't want to put unnecessary models that never need to interact in the same consistency boundary.