I think there is no technical answer to the question why it is not modelled together. I think there are a couple options:
- piping and structural are 2 departments and don't communicate (well).
- there is no money available for the extra modelling work required to get the structures in the piping model.
- the piping is often calculated before the structures. Piping forces are given to structural engineers, but structure flexibility is not passed back to the piping engineers.

Using an axial gapped restriant because there is some stiffness in the structure can give you an idea about the effect on the piping if the structure deflects. However the deflection depends on the piping forces and thus using a support with a set stiffness would be better modelling.