Because it is explicitly mentioned as so in al-Kitaab, or theologians have derived such meanings of it.
So in the mind of the Muslim, do you believe the words of Allah in the Quran regarding the Earth, or the words and images of the Kuffar scientist?
This is where the moderate/secular/normalguy muslim loses and the "fanatic" wins, what the Fanatic says is based off the al-Kitaab and is hence backed by Allah or the Profit.
That is where the buck stops, since Allah will not lie, his words will not be false, and this is how the more saner aspects and people among the Muslims are defeated and submitted to the "true and pious muslims" among the group.