I think I can answer this from my perspective. Currently I don’t have a sect. I was born into a Sunni family and community. Everyone in my city is Sunni, they’re the majority by a landslide.
when I was taught about Shias i was taught they were brainwashed. They killed the prophets grandsons. They repent by hitting themselves. They’re evil.
Some Sunnis have this ideology. They have been fed a narrative which paints them out to be the true followers. Now look at Wahabis. They teach they are the true followers now. It’s a cycle