Why is it so hard for Christians to accept that their religion is little more than the collected mythology of a defunct and irrelevant desert people with no objective reality?

Faith. It's faith.

I lack it, thus see any religion making a claim to access to divinity as little more than an entertaining delusion. I have no issues with someone else believing what they want, mind you. But because I lack faith, there's zero difference between your religion of choice and that of one of the fundamentalist sects.

To you, your faith says that the divine may not be expressed in a literal sense in the holy texts of your religion. Thus, looking at a fundamentalist trying to rectify the many contradictions and impossibilites within it as a literal truth makes them seem delusional.

To them? We're both going to burn in hell because we reject the word of God.

It's all about faith.