Why has no country in the history of Saudi Arabia ever occupied this country completely?
Saudi Arabia is one of the few countries in the world that were not colonized, and therefore it has no independence day. It has a unification day.
This is because of the strong ties that hold each group of people to a certain tribe. Each tribe pledges their allegiance to the head of the tribe, all of which are from the Arabian Peninsula (Saudi Arabia today). Therefore, it was hard for any foreign power to forcefully or softly rule the Saudis. The Ottoman Empire tried to occupy a small portion of Saudi Arabia (along the western coast) but failed terribly in a raging war by the tribes that kicked the Ottomans, who were seen as foreign, out of the Arabian Peninsula.
Once the founding father of Saudi Arabia, King Abdulaziz Al-Saud made a pact with all the tribes in the region in 1932, the Kingdom of Saudi Arabia was established and is now headed by the main tribe: Al-Saud. Despite going through many difficulties throughout history, Saudi Arabia has always withstood each one of them, thanks to this strong social ties and good governance.