The main problem with enabling autosmooth is performance, it’s rather expensive. Particularly when combined with subdivision surfaces it’s not very helpful and mainly makes things slower.
The plan is to add smooth shading and subdivision surfaces to smooth primitives by default, once some changes to the subdivision system have been made to make it a setting outside of modifiers.
https://developer.blender.org/T54943
At that point it may make sense to auto enable autosmooth by default for non-subdivision meshes. Even then we may need to try optimizing it first.