Every Avatar story is unique, Aang had to deal with the loss of his people, Kyoshi and Korra had to deal with the Earth Kingdom falling apart, and Roku had to deal with choosing to kill his friend or not to stop the eventually genocide of the air benders. Each Avatar had hardships and points where they weren't their best but that shows that they are human and separate from each other. They may be their past lives but that doesn't mean they have the same personality or thought process. They also had to deal with different things that were happening in their time. Kyoshi separated her town from the mainland because of all the corruption, Roku didn't want to kill the fire lord because they used to be friends, Aang had to deal with the pressure of learning all the elements and being told to go against the ways of his people to save the world, and Korra had to deal with the Equalist, the Dark Avatar, The Red Lotus, and Kuvira. The only reason we see more personality in Aang and Korra is because we get to see them through their journey on TV and if you read Rise of Kyoshi and Shadow of Kyoshi you would see Kyoshi change too with only Roku being shown when he is needed like in flashbacks or when he gives Aang advice.