A centralized system could be held even by one person. On the other hand, a decentralized system could be by multiple people or departments. There is a big difference.
...because the centralized seats of power can do nothing but protect themselves from threats.
If they are afraid of those threats, they can decentralise their authourities to the inferiors.
And once the decentralize, their power starts to crumble, as each will end up acting more for themselves, moving toward becoming the threat.