And they have not! Both F22 and F33 are significantly slower than say F4, F104, or F106 all designed in the 50s.
Or the SR-71...
Indeed although it wasn’t a fighter; it was a reconnaissance aircraft.
This is true, although it was tested in a fast response fighter role, overlying enemy aircraft and firing air-to-air missiles from high altitude.
Going faster and improving are two different things. Planes have improved immensely since the 1960s, but they don't go faster. Commercial aircraft have much lower fuel use. Military aircraft have things like better radar and stealth.
Fighter planes don't have to worry about cost, noise, or environmental impact.
I believe the SR71 is still the fastest fighter jet?