User blog comment:Daddy Danjer/Why need the Keepers if there is the catalyst??/@comment-4558124-20120509031352

My thinking is that the Catalyst probably has some sort of block that prevents it from directly interfering with the Citadel and the Relays, the reason being that the Catalyst could potentially become the very problem it's trying to solve. If it's an AI (as it appears to be), then it's without doubt a very sophisticated one, and if it were to somehow become corrupted, it could easily bring about the end of all organic life in the galaxy - much as the organic-made synthetics that it's trying to keep in check would do. The keepers and the Catalyst's inability to control the Citadel/Relays are probably aspects of a system of fail-safes, ensuring that only the Reapers (which, for the sake of the argument, we'll assume don't share the same fallibility as a regular, non-organic-derived AI) can mess with their technology.