Last Airbender: Humans learned to Bending from observing animals that could bend the elements to their whim. Bending techniques and movements are unique and representative of these animals, similar to how real-life martial arts took inspiration from the natural world (usually animals.) Bending is thus a discipline that has been taught and past down generation to generation for thousands of years.

Legend of Korra: Humans were given Bending by Lion Turtles, and they just have to punch and kick.

They do try and state that there was a difference between just having the power and actually using it effectively. As it took Wan a bit of training with Rava to do more than just lob a fireball. So you could always say that the lion-turtles simply unlocked the ability in humans, while it was nature that allowed them to truly master it. They just do a shitty job at explaining it.

Even then, it still ruins a lot of the mysticism of bending. Plus, it goes against the ending of Last Airbender where the lion-turtle specifically says his time was before bending was actually a thing.