Just driving to the airport in Denver is nearly an hour for most of the city. It'll take a half hour to get from Uptown Dallas or Frisco or Saginaw to DFW. It's like a half hour to get from Orlando International to any of the Disney resorts. About 20 minutes from downtown KC to MCI. All of this is without any time parking or going through security and assuming traffic doesn't get bad.
Dallas has a train station downtown. Same with Fort Worth. Kansas City Union Station is downtown. Manhattan has several train stations. A lot of cities have a big train station downtown, as many cities were built around the train station. A decently sized train station uses considerably less space than a busy airport.
Which of these seem easier for the people in the city to actually get to and use?
Dallas Union Station: https://maps.app.goo.gl/aMwyguz98hptWPNo6
DFW Airport: https://maps.app.goo.gl/5iUpUoqvUkhJr1D98
Penn Station: https://maps.app.goo.gl/JysFmwFhc3cwqHLC6
JFK Airport: https://maps.app.goo.gl/MHWe4UM4wCk9iL1F7
Don't get me wrong, I agree even door to door air travel will usually be faster when talking about the kinds of distances a lot of US travelers go at and often people act like a train is 0 minutes of time getting to/from the station, but arguing the whole travel time getting to/from the airport and dealing with more security is only 20 minutes is a massive stretch for a ton of Americans. Most people should budget probably an hour before their scheduled boarding time to deal with the about half hour drive, the time navigating the airport, and the time dealing with security. Plus add another half hour after landing to actually get someplace interesting.