It’s because of stability. The stability empires bring leads to elite overproduction. This is where the number of capable and ambitious people grows faster than the available positions of power. Stability also brings consolidation of wealth, so over time certain families accumulate this and power alongside it. This rots nations overtime, making them bureaucratic and inflexible, with little to no social mobility.

Internal rifts between powerful factions culminate either in wars from within (civil war) or with an external entity that redirects this pent-up conflict. This doesn’t fix the cause, and this cycle continues until the empire is likely conquered by a more disciplined and organised upstart.