For the first and simple reason that the U.S. is a democracy (not a meritocracy) driven by a political system that violates the most rudimentary principles of freedom.
Hence becoming the president of the U.S. means you end up herding a little over 15% of the foolishly enslaved into the fold of your populism. Quite a different task than inspiring the remainder of 70% of Americans who, for good or bad reasons, have disenfranchised themselves from the political process. Let’s be clear: you do not represent the people of The United States when you become the President of the United States.
The second reason is that a President of the United States does not have a lot of legislative power, especially not because his rulings must be approved by two other legislative bodies (the House and Senate), each in a state of political delusion. Plus, be held accountable to the downstream optimizations of an outdated constitution (a great feat of its time, yet at the end of its runway of today’s relativity) offering little or no opportunity to reinvent this country upstream.
I would not need to think twice to decline such ethereal responsibilities, but I am more than happy to offer my expert advice our Presidents so desperately need to build a theory for humanity that determines what can be discovered, and improve this country and humanity fundamentally.