[leetcrew]

they're not? it just turns out that the most efficient cruising airspeed for passenger jets is in the upper end of the subsonic range. most people can't afford supersonic travel, and the ones that can overwhelming prefer to spend the extra money on a more comfortable subsonic flight.

interestingly, extremely high speeds turn out not to be that useful for military aircraft either. the fastest production fighter jet of all time, the mig-25, was introduced in 1970. the f-22 has a considerably lower top speed, and it's not because they lacked the funding to make it go faster.

[aidenn0]

Also, we don't really build dedicated interceptors anymore; the F-106 is the last US interceptor I can think of.

The F-15 was designed primarily as an air-superiority fighter (and in fact misinterpretation of the Mig-25 design was motivation for increasing its maneuverability; the Mig-25 was a dedicated interceptor).

[edit]

In general the Soviets seemed to really like interceptors, the MiG-31 is arguably faster than the Mig-25; altitude and willingness to destroy the engines affect the calculation.

[lumost]

This is largely just military doctrine. Stealth was determined to be a better bet than speed against SAMs and other jets. With improved signal processing techniques and better standoff weapon, speed may be the better bet.