Not only are we to cheep to fly fast but we've also banned supersonic flight over most "civilized" land masses. There are only three or so US mainland airbases where it is legal to break mach 1 due to the fact that the voting public doesn't want to listen to sonic booms. Thanks to relatively new "shaped sonic boom" technology it is possible to massively reduce the acustic profile of supersonic flight but the damage is done. We're too cheap to fly fast and the technology which we need to fly fast still doesn't come cheap.