Why did Monkey D. Garp refused to become an admiral?

Given how strong he's been in the series thus far, and his reputation as a hero, it might be surprising to see that he's still a Vice Admiral after all his years of work. But Chapter 957 of the series reveals that Garp doesn't want to be promoted to Admiral because then he'd have to work directly under the Celestial Dragons.

As Sengoku explains in the chapter, Garp originally made a name for himself as a naval hero for surprisingly allying himself with Gol D. Roger in order to take down the threat of the Rocks pirates. Sengoku reveals that Garp never wants to talk about this fight because not only did he work with pirates, he did so in order to protect the Celestial Dragons. This strongly opposes his moral code, but he's never been in danger of losing his position for insubordination for refusing to serve the Celestial Dragons.

Becoming an admiral means that he'd have to work under them directly, but his accomplishments as a hero keep him from being killed or losing his position in the Navy for openly rejecting their favor. Fans have seen this side of Garp before, like when he pretended to lose to Luffy at Marineford or was revealed to have taken Ace in after Gol D. Roger's death. And now we know why he can do these things without major ramifications.

Garp's stature allows him to be openly brazen, and it seems that he's fine as long as he's a weapon for the Navy. But that also begs the question of just how much of a threat the Rocks Pirates once were if Garp has essentially gotten a free pass for saving the Celestial Dragons long ago.