...And most previous empires were in Eurasia, where a ground invasion of the imperial center was possible, or in Central/South America, where they tended to get ended by volcanic eruptions that would wipe out the imperial center, or cause volcanic winters leading to starvation, or get ended by Spanish who were several tech levels above the locals and had immunity to diseases the locals didn't.
Also, well, Mongols brought down a lot of empires in their time, and we don't have the same issue with being open to the Eurasian Steppe lands.
The US is vulnerable to none of that, unless you count Yellowstone.
The US is uniquely positioned, geographically, technologically, and socially, to have nearly none of the vulnerabilities that have brought down previous empires in the Old World, or the vulnerabilities that brought down the Meso-American empires.
America is exceptional in so many ways, and is an exception to historical trends that only the Mongols were before now.
We do not have to be constrained by the fates of other empires, because we do not have the same geographic or environmental factors that led to the downfall of most historical empires.