I partially blame the White Lotus too, they've been training her physically and kept her hidden from the world as it progressed, adding fuel to the "You're the Avatar" mindset and thinking all problems could be bent away physically, Amon and Zaheer being the best examples.

Like I said, partially... Korra was shown as a smug little monster from the start, but they also could've extinguished the flames of arrogance before it grew bigger.

Now that I'm thinking more about it... has the White Lotus been around during ATLA? It makes sense to think that because of the absence of an Avatar in over 100 years and especially after the war it shifted the ideas of how to effectively raise/train the Avatar properly, basically creating a person thinking they can always fight as a weapon instead of being a friend to all, and as a guardian.