You make a good point. It all comes down to ego. Once someone has a belief that they identify with, a threat to their belief becomes a threat to their ego which in the fickle human mind equates to a threat to their very life. Absolute defence is then called for.
To lose a debate feels like death. The mind throws up every trick and diversion it can, like a mouse running from a cat, it become a life of death situation - in the mind of the believer.
In all this mayhem the reality of our precarious existence and the search for understanding is lost and obscured by insults, counter moves and rhetoric.
A sad waste of life itself...