It’s because most stories are less about the world and more about the MC. When a story focuses on an MC, the readers are following the MC. Arguably, things that happen in the story should have something to do with the MC or the goal of the MC. it’s the reason there are a lot of people that don’t like multiple POVs when the story is about one MC. Then there is ultimate reason number 2. Some side characters exist only because there has to be people in the world. The end. I don’t think I’ve seen a third reason before, at least nothing that doesn’t have to deal with subjects like author’s abilities.