I mean they haven’t just dismantled entire agencies yet. The closest they’ve done is lean on a pretty basic idea - the rule making power those agencies have is power that belongs to Congress and is narrowly delegated to the agencies by Congress.

This means that such agencies cannot make rules that contradict legislation passed by Congress and can only make rules within the span of things delegated to them and no further (because Congress only delegated the power that Congress actually delegated and nothing more, even if it is related or feels like it should fall under their remit based on the name of the agency). Hence why the FCC can place controls on the content of radio or broadcast TV but has no say over the content of cable TV or streaming services - their power over the former is tied to their control and licensing of the use of the airwaves as a public commons (which they were delegated pretty broad authority over) which simply doesn’t apply to cable or streaming.