Italy alone used to be a dozen+ independent city states. Divided the world 200 ways seems like a lot unless you look at how much it's been divided historically. And in comparison, that's extremely centralized.
Stealth, space supremacy, global surveillance, carrier groups, strike wings, nuclear arsenals, reaper drones and all the other crazy battlefield robot tech in service or development can cut to pieces any obsolete nation state military, let alone some rebels armed with cheap 3d-printed guns.
So why isn't the US (which has all this tech) winning in Afghanistan over some rebels with AK 47s? Likewise Vietnam after literally a decade of fighting.
Weapons superiority is one factor in winning wars. Likewise, to address the gp's point, the history of political organization is partly driven by military technology.
And the current struggles of the American military only result in hundreds or maybe thousands of American deaths, which historically, would be a mere rounding error. Afghanistan, Iraq, Syria, et. al. are a mess, but they don't come to a significant human cost to America.
The United States and its voting populace doesn't really care about whether or not developing countries are wartorn, and quality of life is decimated—America and other states have completely destroyed the backbone of several societies in the Middle East and caused the deaths and relocation of hundreds of thousands of innocents, but you don't feel the pain of that when you live states-side. So the country has little incentive to quickly resolve these conflicts besides the bad PR; ongoing conflicts mostly buy time for protracted, proxy diplomacy with other major powers to lay claim to natural resources, and aren't viewed as conflict with a tangible human cost.
The number of states waxes and wanes throughout history, there's not a clear trend.
What's interesting is to consider that while the world is very centralized in the sense you described, "day-to-day" operations are even more decentralized than ever. That is, (very generally speaking) order is established and maintained, and people go about their lives without huge, burdensome micro-management from "above" in the important aspects of their daily life.
It probably turns out that the most consequential decision-making affecting the modern person's daily life is happening very locally in the social & spatial/temporal sense. So, in that way, authority is effectively decentralized.