Because: Allah gave us the Koran, and that's all we need.
Ans it's almost true in a way. Because if the Koran taught them a philosophy of conquest, they don't have to develop the products of civilization, they just have to take them over.
The great achievements that are said to have come out of the Islamic world were made either by non-Muslims who happened to be under Islamic rule, or by heretics who usually had little interest in Islam. Scientific discovery tapers off dramatically as Islam asserts dominance, until it eventually peters out altogether.