Frequency stalled because we stopped being able to efficiently cool the CPUs at that point. Heat output is proportional to power consumption, which is proportional roughly to the cube of frequency (power is normally proportional to the square of frequency, but to drive higher frequencies, you often need to drive up the voltage--which also has an effect on power).
That can't be the only reason, with every shrink power requirements have been reduced.
P4 had up to 115W TDP. The same frequency and raw power could probably be achieved on 15W today. But you can't get most current CPUs to run stable beyond 4GHz base clock, even with liquid cooling.
Not sure what cpus you're looking at but everything in the desktop space is over 4ghz nowadays. My last cpu was at 4.6ghz its entire life and my 3900x stays at over 4ghz on all 12 cores when doing a render.
Yes, I know, but it never actually goes below 4.0 for me on all cores when doing a render.
The 4.6 was referring to my 4770k which was at 4.6 on all cores all the time.
