The other reason business travelers like flying shuttle routes is the ability to go every 30 min to an hour. It’s a lot easier to do that with airplanes than trains.
Look at the standby list for any DAL-HOU during peak times. It is full of business travelers that wrapped up early and have the luxury of going home on an earlier flight.
I agree with Zap that the TSA has done a lot of damage to shuttle routes, but most business travelers are packing light and able to use pre-check. They bypass the counter and head straight to the gate and get in line. On a normal travel day that takes about 15-20 minutes at either of those airports.