It's easy for the Man of Steel: Whenever he's tired of saving the world and needs to retreat to his Fortress of Solitude over on the North Pole, he simply takes off and flies there, on his own power. The rest of us are not superheroes, and for us travel to remote places means we need to board a bird of steel (or, more accurately, some aluminium alloy).
Poster by Hamlet Au Yeung for Do The Green Thing
You can't deny that airplane travel is fast, but it also has a huge carbon footprint. It is true that any given scheduled flight / train / bus will depart whether or not you bought a seat on it. This argument certainly holds for travel on Amtrak trains, which tend to have a low occupation (outside of big travel holidays like Thanksgiving). So the marginal carbon emissions - the emissions due to the extra person travelling on that half-empty train - is quite small.
But airlines have removed from their schedules those flights that tend to be half full, and consolidated them with other flights, so most airplanes are filled near capacity these days. This means that on the aggregate, the more of us are looking to fly a segment (say Boston to San Diego), the more flights the airline will schedule on that segment.