Why can't we just agree to disagree?

Well, that's humanity. And part of the purpose of government is to allow us to "agree to disagree" without violence - which by and large, we do. As to why they mud-sling, that's just as simple - it works! Why does it work? Because a significant portion of us use that to go by when we vote, rather than study the issues and people - that would be too much like work! So we get the government we allow.