Because that's the way Oda uses to show luffy is strong. 
Base Luffy with Haki = Has great Stamina , Fought Seraphim for 4 hours. 
G2 with Haki = Maybe 2 hours. 
G3 with Haki = (Continous Usage) maybe 1 and ½ hour
G4 with Haki = 20~30 Mins (depending upon which gear 4, Snakesman or Boundman)
G5 with Haki = Lower than 10/15 Mins. 
Luffy strength lies using a lot of Strength at once , His G4 had A description of Using a full blast of Haki and Devil fruit. 
Means is Base Luffy can Fight for 4 hours , Haki is distributed Equally injured 4 hours. 
In g4 , All the haki is used within 20~30 mins. 
In g5 , it gets even worse , all the haki in 10~15 mins. 
Luffy is completely rendered useless then. 
That's why I say Luffy is the Psuedo Top tier of Top tier segment , 
All other Top tier needs to do is Make luffy run out of time and win easily. Kizaru did Exactly the same without Playing any offense. 
Kaidou did the opposite , Went all out bonked Luffy. Making him lose gas for G4. 
Every Top tier is Practically winning against Luffy as of now.