system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do countries, such as Saudia Arabia, rely on being sold weapons, instead of making them themselves?
gpt
You can buy a cabinet at Ikea for $30, or you can take up carpentry, buy the raw materials, and spend several days building a cabinet that ends up looking worse than the Ikea cabinet. 

To expand on the metaphor, complex industrial development is more difficult than you might imagine, and high end military equipment is some of the most complex manufacturing there is. 

Other countries have already specialized in making those things, it is much, much cheaper to buy from the countries that are already making it than to try and build your own industry from the ground up.  Saudi Arabia can sell oil and buy weapons and get, say 250 f-16s. If it tried to sell oil and develop its own fighter jet, it would take a long time to do so, they would end up with a less capable plane that was less reliable, and would have 30 Saudi knock-off F-16s for the same costs as buying 250.