The Object type defined above is inappropriate for re‐use here, because Objects can contain fields that express circular references or references to interfaces and unions, neither of which is appropriate for use as an input argument. For this reason, input objects have a separate type in the system.
Since posting it, I found that circular references actually are acceptable, so long as those are nilable (or else it would declare an infinite chain). But, still there are other limitations (e.g. interfaces) that seem to necessitate a separate type system for inputs.