Why dont space marines rule worlds?

Outside of chapters claiming a world as a home world and usually being in charge of said world why dont space marines who are more aged retire from front line service and become planetary governors? Most marines are heavily indoctrinated and thus more resistant to corruption or bouts of petty human emotions. They live VERY VERY long lives and could put plans in place that are generational rather than just a few years. They are learned individuals whose long lives of service have let them see all maners of threats facing the imperium and thus probably make them good at spotting the early signs. Plus they would share brotherhood with other retired marines of nearby worlds and would be more likely to create efficient systems to boost productivity and be more willing to assist each other. I would think that rather than letting an old marine who isn't quite as quick as his fellow marines just die in service putting a few here and there as governors would be more beneficial to the imperium than letting them die in service or am i missing something that makes this impossible?