It is not a huge mystery that many of the ancient empires and kingdoms lasted a really long time. The Egyptians seemed to last a really long time. Well somebody else did the number crunching, and if you are careful to avoid counting a collapse and place in the same place, the typical empire lasts less than 500 years. Or to be more specific 349.2 years (see link here).
If you start the restoration of the Stuarts in 1660 as a starting point for when Britain became a serious empire (see link here), than they made it almost to the 300 year point. Since than you have had a number of flash-in-pan Empires, Napoleonic Europe, the Soviet Union, the Kaiser Germany, and the Third Reich all failed to make it past 100 years.
But maybe that is simply because the world is more globalized and that leaves less room for powerful empires. The bench mark has been set higher.
Interestingly enough though, there is another trend that has also been shortening: Imperial Downtime. In general the interregnums (down periods) between the collapse of an empire, and the restart of its successor is shortening. The chaos doesn't last very long.
The Qing Dynasty and Its Neighbors (pdf)
Victor Lieberman, Social Science History 32:2 (Summer 2008)
As political, cultural, and commercial ties between subregions grew tighter, as integration grew more normative, the state grew more stable, and successive interregnums tended to become both shorter and less institutionally and culturally disruptive. Although one can debate specific dates, I submit that China’s first major imperial interregnum, the so-called Age of Division following the effective Han collapse in 190, lasted some 399 years. The second, from the onset of severe Tang debility to the Song reconquest of the south, lasted roughly 119 years. The third, from the collapse of the Northern Song to the Yuan conquest of the south, was 152 years. The fourth, from the outbreak of anti-Yuan revolts to Ming accession, was some 17 years, as was the fifth, from the mushrooming of anti-Ming rebellions to the Manchu conquest of Yunnan. The ratio of major Chinese imperial interregnums therefore was in the order of 399:119:152:17:17. In mainland Southeast Asia and northern Europe progressive consolidation began over 1,000 years later than in China. But the same pattern of decreasingly severe and prolonged interregnums—or conversely, ever more secure integration— obtained. Thus in Burma, for example, the ratio of the first three interregnums was 252:14:5. In France the first interregnum, from Carolingian debility to early Capetian vigor, lasted some 220 years; the second, corresponding to the Hundred Years’ War, lasted 116 years; the third, the Wars of Religion, lasted 36 years; and the fourth, severe disorders associated with the French Revolution, lasted less than 2 years. In Russia, which had only two genuine interregnums (those between Kiev’s collapse and Muscovite regeneration and between the onset and conclusion of the Time of Troubles), they lasted 210 and 15 years, respectively (p291-292).
So if the empires are falling faster, the reorganization time is also becoming a lot shorter. Maybe not the length of a halftime show, but certainly short enough that you might live long enough to see it.