Most countries you’re thinking of as counterexamples don’t have an equivalent to Air Force One (= near-complete invulnerability for said dictator) or nuclear submarines with SLBMs (= near-complete invulnerability for said dictator’s hard power.) Nothing a militia does storming US land civil or military infrastructure would matter to a US dictator, any more than it would if a foreign military did so. The US military domestic defence system was designed under the assumption that this is exactly the type of attack a foreign military would try, in fact—convincing US citizens (and members of the US military!) to foment a coup, the way the US foments coups in other countries.

Basically, the US president is in the constant implicit position of being able to hold the entire US hostage from a flying doom fortress† like some kind of supervillain. The only people who can really stop a “crazy president” scenario are people either high up in the Secret Service or the Joint Chiefs of Staff, who would 1. Have the authorization to be in the president’s presence during DEFCON 1, and then 2. Have the personal trust/authority to tell the president’s bodyguards to buzz off for a minute.

† The “doom fortress” part applies more to the subs than to AF1, but if you treat the two as a unit, it works.