Because the Saudis have a ton of money to spend; more than they can spend locally.
Arthur Jensen, NETWORK (1976)
Spending all that money locally (and they try very hard to with welfare schemes and building new cities like Kingdom City and whatnot) will just spike inflation, since the economy can only use money so quickly. It's what happened to the Spanish when they destroyed their economy by mining all that colonial gold and silver. https://www.britannica.com/topic/history-of-Europe/Prices-an...