I suspect it may be biological. As social-creatures, I don't think we are wired (by default) to ignore people who disagree with us. From my non-expert perspective, not being oblivious to people who disagreed with us on certain things may have served well from an evolutionary perspective.

An ad absurdum case: would you find it easy to ignore people who thought drought management laws were a joke and should be ignored? Maybe it depends on where you live.

What about people who disagreed that rape was a bad thing and should be punished?

I know these are extremes, but I think it's reasonable to say that there is a spectrum of disagreements we are willing to tolerate. Not to mention that our recent history is filled with some rather deadly disagreements...

Historically, I image those who ignored their brand of Brutus may have received the Darwinian knife to the back.