Inbreeding is very common among Muslims. For Muslims, the marriage is not for love but for business and cementing family ties. The cousin marriage is preferred because that keeps families together. Families already know each other. Muslims see this as making the family stronger. It is like 2 companies merging to make a bigger company.
Also, it is very common in Arab countries to exchange daughters and sons in marriage. Like A's daughter and son marry B's son and daughter. A and B may be cousins as well. In many cases the children are pledged before they are even born.
There are also cases where cousins may exchange their daughters. For example C and D are cousins. C marries D's daughter and D marries C's daughter. C and D need not be cousins, they may be friends or business associates.
Wow, so glad my family believes in marrying for love.
Though, in Europe for a while, this was common too.