Here are two paragraphs addressing those questions:
1. It is absolutely in the country's best interest for the Constitution to be the supreme law of the land. This ensures that neither the President nor Congress can overstep their authority or violate the fundamental rights of citizens that are enshrined in the Constitution. If the President or Congress had the highest power, they could potentially abuse that power to serve their own interests over the people. The framers of the Constitution intentionally created a system of checks and balances, with each branch limiting the others. The Constitution reigns supreme over all branches of government and acts as the ultimate protector of liberty.
2. It is fair and appropriate that the Supreme Court has the power to decide what is constitutional. The Court acts as an impartial judicial body that is isolated from the political pressures faced by the President and Congress. If the President or Congress alone could decide constitutionality, they could simply declare any of their actions constitutional to amass power. The Supreme Court provides an essential check to prevent unconstitutional laws or actions. Of course, reasonable minds can disagree on interpretations of the Constitution, but the Court's power of judicial review has protected fundamental freedoms since the early days of the republic. Overall, the Court does not hold too much power, but rather an appropriate amount to serve its critical purpose in the U.S. system of government.