Because the expectations are that Luffy has yet to reach the level of the true GOATS, Pirate Kings, Legends among legends. There are 3 beasts that were introduced in chapter 1 (Shanks), chapter 50 (Mihawk, silhouette in 49 but yeah) and chapter 100 (Dragon).
Chapter 1 - 50 - 100. Shanks, Mihawk, Dragon.
And then later we had (presumed previous WSS and official Pirate King Vice Captain) Rayleigh at 500.
At 1000 we had the GREATEST fight of Roof One Piece with Top 5 supernova vs Yonkos Kaido and Big Mom.
Those chapters mean something. 1500 should be epic and with the way things are going who knows if we won't reach 2000. Could be 1500 = One Piece, 2000 = Beat WG.
Unless we expect Luffy to just steam roll from here on there should still be another level that is exclusive to Shanks, Dragon, Mihawk + a few other possible candidates like Akainu, Kong, Imu, CP0 boss, Blackbeard and perhaps some of the Gorosei among the living. Obviously Roger, WB, Xebec, Ryuma, Joy Boy should have had it as well. Garp, Sengoku, Rayleigh could have had it but not able anymore to use it due to age.