It's because whatever you're running isn't stressing the processor.
The processor scales depending upon load. Its intention is to give the same feel for overall response by increasing clock speed as needed and to throttle back when not, saving battery life.
Compared to a desk/laptop with a constant-frequency processor that just bleeds power and heat when it's not really doing anything, this is a superior approach - especially for a mobile device.
PS - you've got a ton of stuff running in the background, it's a Linux system - otherwise your processor frequency would drop even lower.