Because the Amercians had bad experience with kings, they didn't want another king, so they made the ``Checks and Balances"' system. So no branch of the government is too powerful:
the President can veto laws made by Congress and can appoint judges of the Supreme Court.
The Supreme Court can declare laws (of Congress) and actions (by the President) unconstitutional.
Congress can override the President's veto and remove him. Furthermore, Congress can remove judges of the Supreme Court.
So each branch controls the two other branches.