At the hypothetical level, a neurosurgeon has opened up a path from the scalp to the brain, and it takes time for that pathway to heal and restore the natural barriers (especially the skull). Depending on the procedure, a piece of skull removed might not be replaced or the opening covered over (BTDTGTTS, or rather, hole in the head) or the surgeon may cover over the hole with plastic. And then the scalp is sutured or stapled.
Whatever the particulars, the natural protection from infection is compromised during the weeks of healing time. If a neurosurgeon knows a patient's home is rat- or roach-infested, the reluctance Carson mentioned would be professional and rational.