Also because it's so slow. You're tying up a lot of personnel on the train, as well as capital equipment, for multiple days, rather than hours for a plane. With the same personnel (which cost about as much as pilots/cabin crew), you could run ~10 equivalent flights. Capital equipment is cheaper for rail than in the air, but the portion of rail capex/opex (either directly, or usually via fees to freight lines) is also high.

This video is a good analysis. https://www.youtube.com/watch?v=fwjwePe-HmA