Because the US has taught its military and other branches to respect the balance and rules from the constitution and precedents. Trump, like facists before him, exploit these rule followers to gain power through populous appeals, lies, and subterfuge. Then they push the boundaries to further consolidate power and accomplish their goals, however poorly thought out they may be. Take for example Trump's talk of a third term. He's already tried to start a coup when voted out the first time. He certainly won't stop if there is any chance he'll face real accountability.