it's not how things work ... I could've make the same argument for Dark King or Kata if we didn't knew they are CoC user
Garp being CoC user and a fucking great one at it make sense for plot , his story arc and general power level
What case for Dark King? The guy was Conquering the Grand Line with Roger
Katakuri is the pride of Charlotte Family & helps his Mother in her Conquering
CoC isn't exclusive to Leaders only, what matters is that they have Ambition & Strive to Conquer something
Even Hancock conquers Men & is extremely bossy for example, it's in their Character
But Garp is just a Soldier, he have no Ambition, nor is he part of someone else's Ambition
The guy couldn't even fight in MF & only joined at last minute
While Luffy on the other hand didn't hesitate to Punch him or Koby or anyone in his Path
Not because Luffy doesn't love them, but because he is a Conqueror
Actually even if we go by only Leaders can be Conquerors Logic
This would mean CoC Users only accept to be Henchmen when their Leaders are also CoC Users
Rayleigh follows Roger, Kata follows Big Mom, Oden followed Roger & WB, Ace followed WB, Zoro follows Luffy (And that's why Oda added Scene of Zoro saying that he would have became Captain if Luffy doesn't have CoC at least, it was a Major Hint that CoC Users can only follow other Users like them)
They can retire after of course such as Chinjao letting his Son lead or Sengoku retiring from Leadership but they Ruled & Conquered once. Garp on the other hand never Ruled, have no Ambition & isn't following anyone with CoC, so all three conditions aren't found in him
Dragon on the other hand is a Leader so he could have CoC, Mihawk is alone so he could have CoC, Beckman is following Shanks so he could have CoC, Yamato is Daughter of Kaido & might join Luffy so she could have CoC ... etc But not Garp