Because their union is not stable. Remember what happened with OIC. What happened with United Arab Republic (a union of Syria and Egypt). Study why they failed while EU didn't
en.m.wikipedia.org
But when you and me understands the importance of this why not the leadership give up their differences for the sake of survival. Especially after watching infront of their eyes what happed to Iraq and Libya. Who is actually making them not realize their plight and who is ensuring that historically arched enemies are working together to dismantle the powers of their adversaries?
Do not you think the idea that if some UAE, Saudia and Pakistan make one belt they can take on the whole world eventually because they have both wealth, atomic power and man power. Especially when they see that the west blindly support anti islamic things such as france and especially ghaza.
Think without a bias. There is a superior power who is not making us realize what we are doing to ourselves because our goal is to earn this world. The path that you have suggested is needed but if the intention is to earn this world then the punishment in this world is our hearts are filled with hatred and fear of our enemy and no matter how many good ideas is on the table we will not be able to implement them. This has to start from every individual because leaders comes from within us.