I've always wondered why they can't fly at normal airline speeds until over a rural area or an ocean before accelerating to sound barrier speeds.
Because geometry and engines which are efficient at very high speed are extremely inefficient at low speeds, so you want to avoid subsonic flight as much as possible.
Variable geometry (mostly variable-sweep wings) bridge that issue, but they're weak points, and significantly increase the weight and complexity of the machine, to the extent that Boeing's competitor SST (the 2707) abandoned variable-geometry mid-project as way too heavy and still not efficient enough, reverting to a more classic fixed delta.
good supersonic performance and high subsonic efficiency are essentially opposing goals - a big wing that generates lots of lift is more efficient at slower speeds, but obviously becomes problematic at supersonic speeds. Also, drag is highest near the transsonic regime, and fuel burn is nonlinear, it's partially a function of time, so "getting there faster" can actually burn less fuel than having to run the engines for longer.
There were few countries were you could go supersonic over land: Russia and Saudi Arabia comes to mind.
And when BA last flew one of its Concorde from NYC to Seattle, it flew supersonic over Canada.
https://en.wikipedia.org/wiki/Concorde#British_Airways
The final flight of a Concorde in the US occurred on 5 November 2003 when G-BOAG flew from New York's JFK Airport to Seattle's Boeing Field to join the Museum of Flight's permanent collection. The plane was piloted by Mike Bannister and Les Broadie, who claimed a flight time of three hours, 55 minutes and 12 seconds, a record between the two cities.