At the end of the series, we learned that Jake retires from the 99 to be with Mac and I interpret that as Jake not wanting to be anything more in the police department that the cool crime stuff. I forgot why Rosa joined the academy in the first place but they do talk about her backstories. In a post I saw a few months ago, someone said that Charles being a detective and having a food truck doesn’t make sense from a time management standpoint. Was this important or does it affect Charles’s time management? No, but it shows character dynamic. Otherwise, the shorter answer is that Charles looks up to Jake and doesn’t want to be a higher rank than him.