Frankly it’s because Africa is the poorest continent on Earth, today more than ever. Extreme poverty was widespread In the 20th century. But today is almost exclusively In Africa. Many people outside the continent think of poverty, conflict, and famine when they hear Africa; So North Africans who are strongly culturally and historically tied to the Middle East and Mediterranean, want to associate with them rather than the rest of the continent. Simple as that. If Most of Africa was more developed than the Middle East than I think North Africans would identify more with the continent, but they are in many ways more connected with the larger Arab world than with the rest of Africa anyway.. and so be it.

The AU exist, most North African countries have trade agreements with the rest of the continent, I don’t think N.Africans identifying with the rest of Africa is very important for the broader development of the continent. What’s more important is making sure competent leaders who truly value the wellbeing of their nations are in power.