because the quality advances.
haki isnt chakra. look at it in 2 dimensions quality and quantity. quality gets stronger as haki blooms and by learning advanced forms your haki quality gets stronger. quantity runs out if you use it extensively.
But quality is related to quantity, haki gets weaker when it runs out.
Quality also affected by exhaustion and stamina loss. Because Luffy's armament was still weaker even after he recharged it against doflamingo, doffy said it's all luffy could do to stand up.
It's the same with the katakuri fight, luffy rechatged, and immediately starts to showcase progress, and it didn't show no signs of weakening even after hours of battle.