Because I didn't know the history myself, tl;dr from Wikipedia:
Ottoman empire controlled the entire area at the turn of the century, to various degrees at various time. Al Rashid family allied with Ottomans. Al Saud family, under Abdulaziz (later, simply Ibn Saud), fought Rashids in the Riyadh region. During WWI, pan-Arab revolt against Ottomans encouraged by British. Ibn Saud mostly avoided participating in revolt to focus on defeating the Rashids, with the help of Ikhwan (Wahhabist) forces.
Pan-Arab revolt failed, but allies won, resulting in creation of independent Arab states. In years after the war, Saud and Ikhwan forces captured and most of modern day Saudi Arabia.
In 1927, the Ikhwan forces attempted to expand farther afield, into British-administered Arab successor states (Transjordan, Iraq, Kuwait). Saud disagreed, and a two year war was fought for control of Saudi Arabia. Ikhwan forces lost, most of their leaders killed, and the Saud family rules to today.
But isn't that just because they are a single-resource export oriented economy? It's Dutch Disease[1], but hundreds of times larger?
Britain passed the economic and military reins to the states
Why are you bringing up Britain here? They weren't a colonial power in Saudi Arabia. The closest they came was Kuwait.
[1] https://www.investopedia.com/terms/d/dutchdisease.asp