Because the cultural institutions on which their societies are based - namely Islam - are inferior to those of other cultures.
Of course, the English population skyrocketed at the end of the 1700s and through the 1800s - one of the most enormous reproductive bursts of any nation in history.
And the English standard of living transformed dramatically in that period - to a height no nation had ever seen previously.
Why? Good cultural underpinnings that motivated almost all of those many children to undertake useful, positive endeavors.
Rapid increase in population does not prevent improvement in standards of living, but rapid depopulation certainly erodes standards of living dramatically.