SRB Thrust Schedule

The solid rocket boosters do not produce maximum thrust at all times.
In part this is to avoid excess aerodynamic forces, and in part this is to
avoid excess G forces. I understand these things.

What I don't understand is the following: At times the SRBs could
produce more thrust yet don't, and at the same time the main engines
are at less than minimum throttle. In other words, the designers could
have had the SRBs produce XXX pounds more thrust, and the main
engines produce the same XXX less thrust, and have chosen not to
do so.

Shifting the thrust from the main engines to the SRBs would result in
an increase in payload since it increases the staging effect. The only
limits I can think of being the throttleability of the main engines and
the maximum thrust capacity of the SRBs.

Obviously, the designers have already thought of this, and chose not
to do so. I assume there must be some practical reason. What is it?

-Thanks