Because if the problem is unchecked power, and any checked power is checked by a power that is checked or unchecked, eventually you either get a loop or an unchecked power, which can then do whatever abuse you're worried about.
How do you figure? “Unchecked” means uncontrolled, not uncontrollable (an important distinction). In a functional social system, different powers constrain one another, which is a kind of loop I guess, but a stabilizing one.