Probably for similar reasons that your average apartment tower isn't portable.
Transporting empty big rockets is already really hard and finicky. Transporting something even bigger and heavier than the largest known rockets is harder. Then you need to anchor it to the ground strongly enough to take the weight and dynamic forces of a fully loaded rocket and subsequent landing, but weakly enough you can move it again later. Overall it's really a situation that benefits from a permanent concrete foundation.
Transporting empty big rockets is already really hard and finicky. Transporting something even bigger and heavier than the largest known rockets is harder. Then you need to anchor it to the ground strongly enough to take the weight and dynamic forces of a fully loaded rocket and subsequent landing, but weakly enough you can move it again later. Overall it's really a situation that benefits from a permanent concrete foundation.