Because labor law says that if you're ready and prepared to work for an employer, you have to be paid whether or not you're actively involved in something. Wait

Because labor law says that if you're ready and prepared to work for an employer, you have to be paid whether or not you're actively involved in something. Waiting around for a ride counts as "working", so you get paid for it. It's the same reason that an employer can't make all their employees clock out when the factory production line stops to fix a problem.