Not who you responded to, but I've listened to a number of interviews from the former CEO (highly recommend the Odd Lots podcast for his interviews and many other supply chain related conversations). A couple concepts, some details may be a bit off and I'm very open to correction.
There is a lot of administrative cost. IIRC, you might have something like 1 dispatcher for every 50 drivers. You have other people giving quotes via phone, decently high turnover for truck drivers, slow loading warehouses that cause a cascading effect when the driver can't complete all the stops planned for the day, and a strict limit on hours worked for truckers.
Some of the pandemic port congestion was truckers arriving at a port to pick up a particular load, but that load wasn't ready right then. Flexport has an app which basically let drivers on their platform arrive at a port, grab any container that's part of their platform, and take that to the destination. This prevented countless hours of idle trucks in ports.