Because they are acting/thinking irrationally. I’m not entirely sure what the solution to that is except to gently, slowly, and dispassionately ask small binary questions to lead them in explaining their thought processes until they’ve explained far enough that they admit the position they hold is fundamentally rooted in irrational emotion rather than logic.
At that point, they ultimately have a choice to make. They can admit they are being irrational, embrace it, and continue to act irrationally intentionally, or they can change their position.
I think this approach to conversation is infinitely more productive than both sides just yelling past each other and neither side actually speaking the same language.