Because they don't have hot reserve. If someone calls out sick last minute they don't want to wait 2 hours for coverage. In my experience there, the only time they called me inside of two hours was at the beginning of my reserve time for the day (5am or 11am depending on your SCR line), usually to cover for the line pilot that wakes up sick or has car problems. If there's an excess of reserves they also like to deadhead you to CLT really early. I didn't mind that because once I was out of base they typically kept adding flying so I stayed out of base.