Amazon does something similar to that here. They will send one truck out early in the morning, and another later in the day, maybe around noon. I have had two orders delivered within a couple hours when both orders were shown as being in the warehouse the night before. One left the warehouse at 8 in the morning, one left after 11 am. both arrived between 3-4 pm.
On the other hand, I had two orders with one scheduled for a Monday, and the other for Tuesday, and they held the Monday package, even though it was in the local warehouse the day before. So they do have the ability to combine shipments if they choose to, even if not requested. I don't think most ground facilities have the space to do that. With fedex, and them saving money, they need to have packages move through the terminals as fast as possible or they may need larger terminals. I am sure they don't want to spend any money 'warehousing' packages even if it might mean saving drivers time. If they have to handle the package more than once out of the trailer before it goes into the driver's truck, it costs fedex money instead of the 'contractors.' Using contractors and having mutiple sorts during the day is ideal for fedex. The contractor has agreed to service an area, and fedex can use much smaller terminals if they can have multiple sorts throughout the day. Making full use of a facility you are paying for 24 hours a day has to be the goal. Just like fast food places added breakfast menus and late night/24 hour service after years in business to make more use of a building that was going unused for 10 hours per day. They had people there cleaning anyway- just let them sell food at the same time.
For fedex ground, multiple sorts makes more sense than larger terminals with all trucks dispatched at the same time because fedex isn't paying for the trucks. Shifting as much as possible to ground save fedex express from needing more trucks and drivers, while ground just pays basically per delivery, whether the contractor needs 1 or 2 trucks per day to service any given area.