Most truck garages are booked out several weeks with appointments for commercial customers. Our local Ford truck garage runs 2 shifts 6 days a week and is booked out at least 3 weeks in advance all year long. Commercial customers are their bread and butter. They need their vehicles serviced and repaired on a regular schedule to stay in business. It's not likely their going to push one of their regular customers aside for someone they've never seen before and will likely never see again.
They probably want the vehicle for a longer time period so they can work on it as time allows. They may start it and get pulled away to complete an in "process job" that has parts on order, or an emergency repair.