CPU frequency governors and remote callbacks

You could find ARM systems that do not implement it, too. And even though you may have 8 potential clock inputs in the design, you might supply them all with the same source clock giving you a granularity of "I can set the frequency of 7 other CPUs, but have to consider myself".

Mostly it's a question of power intent and complexity in system design, more than it ever is capability of an architecture or microarchitecture. Whoever is designing the clock, reset and power controllers is the person to ask ;)