Answer:
The specific soil material models have a variable stiffness that depends, among other things, on the prevailing stress level.
In the analysis of a single load case, this is only impressed on the structure and the soil. No stress level from other loads is taken into account, which might be necessary to obtain and use the correct soil stiffness from the soil material model.
The load case of a live load, for example, will result in different stiffnesses and thus deformations,
if it is applied within a load combination on a structure that is already subjected to the soil weight as well as the structural weight and construction load,
than it would result if setting it as the "first/single" load, which would be done in the analysis of the load case.
Therefore, it does not make sense to analyze the soil with the specific soil material models subjected to the individual loads / load cases if at least the always prevailing soil self-weight is not taken into account.