As many have said, it's because they wanted to give the animations a "storybook feel" likely in response to the lack of budget. They realized they didn't have much to work with, and rather than give up, they decided to be creative with it.
As you might have noticed, everyone has the same voice actor, who is meant to be the narrator, who would be telling this story to the youth of Okoto (That Narrator being Narmoto, apparently)
Like imagine if you were a little kid, snuggled up all nice and warm, ready for bed, and your father read you a bedtime story. All of the characters would sound pretty similar, and any female characters would sound male because well, your dad is the one telling the story.