[Matrixbeast]

I don't hold that to be true. Character development can be subtle or it can be blatant. It's all dependent on how the character thinks, acts, and behaves at the start vs. the end of the series. Wataru in Kiva obviously underwent character development, going from sulking in the tub to actually facing his problems head on.
However, then there's the case like Gokaiger. I understand that with multiple main characters, development will be spread thin. There's cases like Marvelous and Don, who did undergo development, with Marvelous realizing that the treasure he was searching for wasn't worth removing the prior 34 Sentai from existing after watching former rangers helping with evac and Don growing a spine to save his friends, and cases like Joe and Luka, who had opportunity to develop, but for one reason or another, never did.