KSA doesn't have a good defense industry because it has been seeking help from friendly countries in all its wars and the level of threats isn't that much compared to now, so U.S or UK forces in the Gulf can do the job instead of KSA.
During this period all what KSA got to do is buying the most advanced weapons in full packages including spare parts and maintenance for tens of years costing tens of billions of dollars!
Although there is an offset program, but there is no much achievement in KSA's favor compared to the billions of $ spent during all the past years on weapon supplies. When the oil prices were at high levels we had a very poor administrations throwing money everywhere and betting on foreign powers to protect the country and fight for us with no plans to build a strong national army or local defense industry. Miscalculation of politics and choosing wrong allies without strategic planning is another reason.
Anyway, it's not the time for crying over the ruins. The war in Yemen taught us who's our true allies and the ones making alliance with us for a bunch of dollars. These lessons aren't for free as we're paying the cost for trusting others and not depending on our ourselves.
As KSA determined to work on its military industry no one can stop us. KSA isn't a superpower so the challenges aren't that the same ones that faces US or China for example. For the air force, if KSA could succeed in producing spares/critical parts and ammunition for its F-15,Typhoons and Tornadoes it won't depend on the west so much like before. We aren't going to invade the world or neighboring countries, all we need is the local capabilities to sustain the military operations if we get stuck in a never ending war with no much dependence on other countries.
By now KSA produces only %2 of its military needs and it should be %50 by 2030, but that doesn't mean we're not making anything. As
@Saif al-Arab mentioned above, we produce different types of ammos and due to the unrest in the region we have expanded the capacity of production lines into fair numbers.
20mm ammo(they also produce 30mm ammos for Apache cannon and Bradley armored vehicle) :
Parts of F-15 being produced by Alsalam aircraft co (labeled by yellow ) for the conversion of F-15S into F-15SA :
More to see about the company :-
Production of weapons database & management system for F-15 by AEC (advanced electronic co ) :
EW systems :
Upgrade of APG-63(V)1 radar to APG-63(V)3 :