While I can see why someone might think that way, the problem is this: simulating quantum mechanical systems is much, much harder than simulating classical physics. From a simulator’s point of view, throwing in QM isn’t a simplification, it changes your simulation from basically linear scaling to a horrendous exponentially scaling nightmare.

Furthermore, quantum mechanics isn’t really an “optional extra” for our universe, it’s too important, eg none of the materials or chemicals you see around you make any sense without quantum mechanics.