Because of the exact situation we’re in now with Trump. If a scheme like that works and the military follows the order then it doesn’t matter what the law says. If the military doesn’t follow the order, you have to punish the president so future presidents don’t view themselves as playing with house money to try to remain in power.
AFAIK it’s still up in the air if Trump’s stupid coup schemes are covered by immunity but if there ultimately is no way to punish him, why wouldn’t every losing incumbent president try?