Slightly more expensive. (Quite difficult to compare directly, actually, because of the huge variability of "offers" at some times, etc. and it isn't always easy to compare like with like.)
But I think the main reason is that for most domestic flights (with obvious exceptions like London-to-Aberdeen, for which flights are well used), given the much shorter intercity distances over here than over there, the time taken to get to/from airports and check-in times are so disproportionately long, compared with the flight-time, that they actually make the journey take quite a bit longer then an equivalent train-journey which tends to be city-centre to city-centre and therefore more convenient as well as usually less expensive.
In other words, the main reasons are probably geographical ones.
I have a very good local airport, where I live, about 200 miles from London, and there are some reasonably-priced flights to London, but door-to-door, flying actually takes me longer than the train. And 200 miles is quite a long distance in England, in huge contrast to America, of course. I fly to London only if "flying on" somewhere else, from (the same airport) there.