Because religious thinking.

People believe without any evidence, they want to believe though, they fill in the blanks and reinterpret quotes to suit their ideal.

Then they deny there is anything contradictory about their belief because it has become a part of them and an attack on their idea is interpreted as an attack on their person.

Fanaticism 101

Prior example is Halo 5 where people would defend and delete comments about the in game micro transactions. Despite being objectively bad for the gameplay and value, because it's "halo" it cannot be criticised it is god, it is all, it must be defended to the death.

Rather fascinating really how many kids/grown men get sucked into that train of thought and demonstrate almost exactly the same kind of vitriol you would expect from a muslim extremist.