It was written before Dragonball Z existed so they didn't have the convenient framework of "power level" to use. Instead the power of Congress is indicated by the fact that all acts of the government are derived from bills originating in Congress, which the president rubber stamps (or not, which congress can then override), and the supreme court judicially interprets - but only if someone brings suit.

Now the president can do police actions and stuff but it seems like the intention was congress being the branch that had independent autonomy to just do things and get the ball rolling.