Theres no money or pension involved in starfleet. The post scarcity space communism pretty much lets anyone do whatever they like.
Their rank/rate mainly have to do with authority, and it didn’t seem like the chief really cared about being in charge. He just cared about making things work.
He was a fine example of an engineers mindset, along with scotty/Geordi/etc. He was also a damn fine solider and tactician, but he didnt like it. He just wanted to keep things humming, and he did it well.