Why not? It's not like they are the strongest force in the marines either so they don't really fit as guys who have enough strength to be considered the ultimate level to reach.
Originally Posted by Tunder Luffy
Plus marine admirals aren't on the same level as Yonkou, not those we know anyway, Whitebeard was declared as the strongest man on the world and Shanks ended a war with his mere presence.
All luffy really needs is to learn how to control his haki, he already has fighting power and skill beyond amazing for someone who doesn't have haki, main problem is he just can't hit them being logia users and considering he has king's haki, Luffy will get a serious boost to his destructive power, and speed perhaps as well cause low speed was a problem agaisnt kizaru.
In other words, the day luffy can control his haki perfectly, he will go from admiral level to demi god level xD much like Garp or Whitebeard and Shanks.