A bit stream of consciousness, and not at all comprehensive:
Name a bureaucratic organization that decided it accomplished its mission, closed up shop, and vanished. Can't think of one? That's because over the entire course of history, there are only tiny handful of examples. Contrary to these extreme outliers, most bureaucracies keep growing until forcibly uprooted. This is because they have every incentive to grow. The larger they become, the more power the bureaucrats exercise, and the more secure their jobs become. If they fail, they're almost never fired. If the organization fails, it's almost never dissolved. Instead, failure is generally an excuse to say they were underfunded, and demand more money. That's completely counter to how the private sector works, and rewards incompetence. And since there are no universal measures of success or failure, like profits and losses in a private company, everything is subjective and therefore almost any failure can be justified or excused.
The bureaucrats' lives are also focused on the organization, while their checks and balances, like public opinion, tend to be intermittent and relatively uninformed. As a result, there's an incredibly heavy bias in favor of the bureaucrat's excuses and rationales, when that even becomes an issue. Because most of the time, they just go about their business in relative obscurity, with no checks. This is a more generalized form of the concept known as regulatory capture.
Another reason is the nature of states. Larger states beat smaller states, so out of self-preservation states tend to grow until they reach the limits of effective governance. This has increased over time, as methods of communication and projecting force have improved, though it's not always a straight arrow upwards (Roman roads and organization were more effective than anything until the late medieval period.) But massive modern states are the result of sophisticated infrastructure.
A corollary is that sophisticated infrastructure allows a central state to exert not just power over distance, but to usurp more and more tasks that used to be handled locally. When there were no phones and Washington DC was many days ride away, governors had to make most decisions on their own. The same is also true with governors and mayors. The decline of states in favor of federal governments, and local governments in favor of state governments, has been accelerated due to airplanes, phones, and computers.
Rulers are also individually incentivized to grow their territory, because prior to the widespread adoption of capitalism, nations didn't get wealthier over time. The most practical way for most rulers to increase their wealth was to increase their territory, so they ended up with more peasants producing more crops. This was done by war, but also by marriage. This is why primogeniture and marriage within the same extended family were so important, because they maintained or increased their holdings. (Look at the Hapsburgs for a good example.)
This consolidation is a historical fact -- in the 19th century, there were more independent states within the current boundaries of Germany (300+) than there are in the entire world, today (less than 200). And while nation-state borders have been relatively stable since WW2 (with a few major exceptions like the collapse of the Soviet Union), the era since has seen the rise of global organizations that have taken over some state responsibilities (the WTO, World Bank, maritime treaties, etc.) and regional blocs like the EU or even NAFTA/whatever it's been renamed. So even where the borders remain static, the tendency toward consolidation continues.