That's the just flight time, but you've also got travel to/from the airport, parking, maybe shuttle bus/monorail, and checking/security/wait time at the airport as well.
So, add an hour for door-door travel to airport, and 2-hr before flight check-in, and now the comparison isn't 3 vs 6hr but 9 vs 12hr, which doesn't sound so worthwhile, although no doubt there are customers for it.
For longer flights it'd be much more attractive, but this is never going to be an affordable service for the masses.
