They do. That's why they regularly have anti-corruption purges and etc., but that is not effective. You can't cure a centralized system's corruption with random controls. Controls are only to identify problems, not to fix them. That is a core rule of Quality Assurance (which we learned from the Japanese).

Also, you don't know if the people in the control functions are not also corrupt. This is a deep, cultural problem inherent to centralized systems, that any problem becomes endemic, so that even the auditors end up in one the game.

Decentralized systems are messier and more chaotic, and harder to steer, but problems from one section tend to not spread as quickly to the others.