I think I know why this phenomenon exists. We as humans are very social animals and we are instinctively driven to be part of a group and not alone. This behavior can be observed in other primates as well that share a good 95 percent-plus of the same DNA with us. It is one of the many features that primates such as apes and monkey all possess, including humans.
We are made to be with other people so that we have the support of other members to obtain food, water, and protection so we can survive in the natural wild world, which we don't live in anymore.
Because of this, we always want to be in a group, not outside of it and therefore, this ignorance emerges in an attempt to sustain participation within a group one actually disagrees with. When we see that our own opinion actually has potential to become a separate group because of other people sharing our beliefs, that is when we finally rebel against the original group that we tried so hard to be in.
This is, in its simplest form, an attempt by our bodies to survive by sustaining an environment where we get support from other humans concerning survival.