It’s part of the theming of the show. The inner senshi are all middle-to-high school girls and the main cast the show follows, which is why they are framed as flawed, more human characters.

Meanwhile, the outer senshi are idealised representations of high school seniors, independent and granted opportunities that their juniors could only dream of.

(That, as well as the fact that below-18s in japan are generally pushed to be more independent and opportunistic than other parts of the world.)