Despite cultivators gaining the ability to fly when they became a Kaiser or reached the level 7 mortal stage, it was an extremely rare sight to see cultivators fighting in the air with opponents who were on the same level as them.
Unless one was a cultivator who could proficiently use wind or some other method to aid their movements, most cultivators stuck to the ground due to the impracticality of fighting in the air.
Even if the heavy qi consumption which was exerted by the flight were to be looked over, the lack of agility was a glaring weakness.
Maneuvering around in the air with dexterity was close to impossible, and that is why so far none of Seth's powerful enemies had hung up in the air and rained down attacks on him, he could only think of how lucky he was that when he first encountered Treuchi, he had Raijin.