Recommend Arabs at War: Military Effectiveness, 1948-1991 (Studies in War, Society, and the Military) by Pollack

There are many reasons; one of them is that Arab forces often lie to their commanders so that they have very poor situational awareness, leading to a cascade of further poor decisions in war.

This is because you can and will be executed for failure.

Unfortunately, you can also be executed for being too competent, which can be seen as threatening to your superiors.

Also, maintenance. And sand. I hate sand.

There are many other factors and exceptions, depending on when and where.