Because the military conditions that led to the rise of countries don't exist anymore.
The modern nation-state grew out of a confluence of technology trends that accelerated throughout the 1800s. Mass-production made for cheap, accurate, long-ranged rifles. These made large armies of relatively untrained soldiers effective, right in time for conscription to fill the manpower void. Steelmaking made mechanized armor impervious to bullets, which also required internal combustion engines to power them, which required oil to fuel them. All of this required supply chains, which meant that political organizations large enough to build complex industrial supply chains and man them could steamroll earlier feudal empires that were still relying on cavalry charges, swords, and muskets. This happened in WW1.
Since roughly 2000 consumer electronics and inexpensive shaped-charge explosives have dramatically driven down the cost of fielding an effective fighting force. A ~6" diameter HEAT round can penetrate any reasonable level of armor, particularly if you can deliver it accurately enough to hit the same spot (defeating composite & reactive armor). Guide it with a microchip and you will hit that spot, and without a whole lot of training. MANPADS can take out far more expensive helicopters & close-support aircraft. Add drones and your $100M fighter jet is no longer assured air superiority, plus you can strike infantry formations from the air.
We've seen this already in those countries that have erupted into war. Afghanistan (repulsed first the Soviet Union then the U.S. using U.S-made weapons), Iraq (insurgency using IEDs), Syria (collapsed into anarchy, multi-sided urban war), Azerbaijan (very effective use of drones against Armenia), and now Ukraine (kicking Russia's ass with Javelins and Bayraktar). IMHO the reason we haven't seen it in developed nations is that it is far more profitable to take people's money than take people's lives. The folks with the capability to field smart weapons & drones are busy getting people to click on ads and fleecing them out of their retirement savings rather than waging war.
Sure, insurgencies don't need fancy jets or million-strong armies when they're fighting against countries that aren't investing much in this specific war. This concept of asymmetric war is literally as old as America itself; it's how it won the revolution: Britain just gave up when it got too expensive.
But it's absurd to think that, under modern military conditions, the strength of an insurgency rivals the military strength of the U.S.. For example, in all these conflicts you mentioned (-Azerbaijan), the U.S. isn't even sending troops (-Iraq, Afghanistan) and our side is still winning with its superior technology and supply chains.
The military conditions today absolutely still favor organized countries. In a future war against such enemy, if the U.S. has much to gain and doesn't care too much about the body count, you'll see Desert Storm levels of dominance. The reason we have all those real big "obsolete" planes is to drop real big fucking bombs like we did in WWII and Vietnam.
(Edit: maybe your insurgency has impenetrable air defenses consisting of thousands of drones with said "microchips". But at some point your insurgency will run out of supply and the U.S., with its "outdated" mass production capabilities and supply lines, will eventually outcompete you and then the bombers will be sent in.)
My point is that the military successes of insurgencies that you're describing are not enabled by a change in the balance of military strength. They're enabled by the fact that there was a significant decrease in the amount of effort that major military powers spent on war, e.g., when the U.S. decided not to irradiate the Korean Peninsula with cobalt. But just because countries don't go all-out anymore doesn't mean they can't or won't. These capabilities still exist and they implicitly influence every international interaction and transaction made with the U.S..
So no, anarchy will not erupt domestically when the U.S. loses some of its global hegemony to China. And the country with its centralized government and military is not yet an obsolete form of organization. Any domestic revolution would be put down easily, assuming there were people stupid enough to try. People will just adapt to whatever change happens and submit to their governments like always.
The modern nation-state grew out of a confluence of technology trends that accelerated throughout the 1800s. Mass-production made for cheap, accurate, long-ranged rifles. These made large armies of relatively untrained soldiers effective, right in time for conscription to fill the manpower void. Steelmaking made mechanized armor impervious to bullets, which also required internal combustion engines to power them, which required oil to fuel them. All of this required supply chains, which meant that political organizations large enough to build complex industrial supply chains and man them could steamroll earlier feudal empires that were still relying on cavalry charges, swords, and muskets. This happened in WW1.
Since roughly 2000 consumer electronics and inexpensive shaped-charge explosives have dramatically driven down the cost of fielding an effective fighting force. A ~6" diameter HEAT round can penetrate any reasonable level of armor, particularly if you can deliver it accurately enough to hit the same spot (defeating composite & reactive armor). Guide it with a microchip and you will hit that spot, and without a whole lot of training. MANPADS can take out far more expensive helicopters & close-support aircraft. Add drones and your $100M fighter jet is no longer assured air superiority, plus you can strike infantry formations from the air.
We've seen this already in those countries that have erupted into war. Afghanistan (repulsed first the Soviet Union then the U.S. using U.S-made weapons), Iraq (insurgency using IEDs), Syria (collapsed into anarchy, multi-sided urban war), Azerbaijan (very effective use of drones against Armenia), and now Ukraine (kicking Russia's ass with Javelins and Bayraktar). IMHO the reason we haven't seen it in developed nations is that it is far more profitable to take people's money than take people's lives. The folks with the capability to field smart weapons & drones are busy getting people to click on ads and fleecing them out of their retirement savings rather than waging war.