This is a simple theory which I believe most people who ever wondered about the subject would have figured out.
Why does it seem like every High Ranking Marine has a Devil Fruit?
I believe this points to the theory that the World Government has a vault with Devil Fruits in it.
Whenever a Marine proves themselves and does something that helps the Marines as a whole greatly such as maybe arresting or killing a large number of pirates either over their career or in one battle, or saving the life of someone important they are given a promotion and a Devil Fruit.
This is why Marines have Devil Fruit powers, especially the Admirals.
This is further proven by CP9 when 2 members of the group were given Devil Fruits as rewards for their work.
There are a few exceptions like Monkey D. Garp but I would say that is, more or less, just them refusing it.
*Theory by Sniper king ussop
