The dynamic Hijri calender?

There is wisdom of Allah in everything that He created, although we fail to understand it many times. I did not understand in teens why should Muslims have a calender that changes every year; and unlike Christmas, we aren't sure when will be Eid. Now I understand that Allah has been fair to Muslims all around the world by commanding the Hijri calender, because a Muslim in any part of the world, gets to experience Ramadan in winters or summers, long days or short days. Imagine, if it was a static calender, one part of the earth would have Ramadan in summers while others in winters, some will enjoy fasting on short days while others on long days. Hijri calender makes it fair for everybody to see all colors of Ramadan, the easy and difficult ones.

Another question that bothered me was; why the weather is so harsh in Arabia, and lack of rain and winters make it so monotonous in the western part where Makkah is. It would have been great to see regulars rains and winters here, but at the same time it would have been so chaotic during Hajj when millions of pilgrims gather at one place, and men among wrap themselves only with two unstiched sheets of white cloth i.e. ihraam. Rains would have caused logistic nightmares and winters would have been unbearable, especially for the elderly.

Another question was, why Allah chose only Saudi Arabia to be send His last and final Messenger, Prophet Mohammad (peace be upon him). I found the answer of the same in the book History of Islam by Akbar Shah Najeebabadi. He argues that different parts of the Word had ancient heritage, monarchy, cultural pride etc, while Arabs did not have any such historic influence, that could have undermined the message of Islam in some or other way e.g. the Arabs could be proud of their race, rather than Islam or non-Muslims could simply argue that these people have always been very creative and intellectual so one person among them invented a new religion.