Add the fares to the fact that there are some situations where getting in the car and driving from one city to the other can be quicker than flying.
Take Dallas and Houston.
If you are starting from downtown Dallas, you can
1. Drive to DFW airport which takes 30-60 minutes depending on the time of day (unless you are flying SW in which case you have about a 15-minute drive to Love Field).
2. Let's say you are one of the good guys who does actually report to the airport for check-in 1.5-2 hours prior to departure. (DFW, you are now up to a minimum of 2 hours. Love--1.75 hours.)
3. You departed on time for your 45 minute flight to Houston. Including your taxi-in and deplaning, let's say 1.25 hours. (DFW, 3.25 hours; Love, 3 hours.)
4. From IAH or Hobby in Houston, you have a minimum 30 minute drive into downtown--though some Hobby devotees say 15 minutes, that would be at 2am.
So, Downtown Dallas to downtown Houston something in the 3.5-4 hour range minimum. With no allowance for unusually bad traffic tie-ups (a common occurrence in both cities), weather delays (very common in high summer), or ATC holds (it's a busy corridor).
OR
You can get in the car in downtown Dallas, drive 3 blocks, get on I-45 South and be in downtown Houston in 3.5-4 hours. AND, you have saved the cost of the plane ticket and the rental car you would need at your destination. I know a number of people in both cities are opting for the drive these days.