Dictatorships have dictators. The power flows from that one person (think Hitler or Stalin), the military is probably loyal to them rather than other state apparatus and there tends to be no codified route to succession in the event of their death.
China is different in all these cases, even after the significant moves by Xi to consolidate power. You could argue that I'm the days of Mao there was a dictatorship in place but things have radically changed since Deng Xiaoping took the helm.
The President of the USA is the commander in chief. The military is loyal to the Commander in Chief until the President changes. What happens when the President never changes. If you want to say “they are actually loyal to the constitution”, if that were the case, they would have mutinied a while ago.
