Over the years, I've seen many people be confused as to why Sora and Riku had to do so much more for their Mark of Mastery than Aqua and Terra.
Aqua and Terra trained their whole life for their Mark of Mastery. Sora and Riku didn't. Sora and Riku's Mark of Mastery was also their training. Their Mark of Mastery was so much more difficult than Aqua and Terra's because they had to make up for the years of training they missed.
The Mark of Mastery and becoming a Keyblade Master isn't about whether you can save the world or not. It's about being traditionally trained. For example, becoming a black belt in Karate has nothing to do with whether you can save the world or win a fight or not. It has to do with if you have mastered the traditional skills you have been taught. The title of "Keyblade Master" is nothing more than a formality.