Why can airliners not fly as fast as fighter jets?
There are 2 big reasons:

Speed costs lots of money. Fighter jets can go much faster than passenger jets but they spend most of their time cruising as the same speeds because it is more fuel efficient. Faster airplanes need more exotic materials and would be more expensive to build. The engines would need to be bigger and more powerful and would cost more, and to go very fast uses a lot more fuel as air resistance increases exponentially with speed. Concorde tickets were very expensive not because of any sort of exclusivity, but because that's what it cost to operate it. Newer technology could probably decrease that somewhat, but it would still be very expensive. Air forces can operate fast jets because they have the funding, airlines have to make a profit
Passenger jets fly pretty close to the speed of sound already, to go much faster they would need to break the sound barrier, which creates a very loud sonic boom that disturbs people on the ground and can break windows in some cases. Supersonic travel over land has largely been banned because of that, so unless you are over water you can't go any faster anyway