Why are aspiring fiction writers cautioned to maintain a consistent point of view? What would cause anyone to write with an inconsistent point of view?

If people who did not have multiple personality were to try to write novels, they would not have any problem in maintaining a consistent point of view. Since they have only one personality, they have only one point of view. All narration and every character's thoughts come from that same, single source.

In contrast, aspiring novelists who have multiple personality will have two ways to make the mistake of an inconsistent point of view.

First, first-person speakers, if aware that they have alternate personalities, may inadvertently reveal that awareness by slipping from “I” to “we.”

Second, the characters, who, like alternate personalities, have minds of their own—their own, independent, points of view—may get away from the writer personality, and insist on expressing their own points of view, even when it is inconsistent to do so. As Toni Morrison said, you have to control your characters and remind them whose novel it is.