Well differences apart, USA military equipment is still most reliable and modern in the world far ahead of what China and Russia are producing.
That's debatable but I think that would be off topic here.
Gulf countries from day one have not focused on developing military themselves rather they still rely on USA for all there defense. USA is in every gulf country for there defense like Doha, Kuwait, KSA. You cant just have a strong defense by buying expensive weapons from west, it takes years of hard work and indigenization.
As I said things are very complicated. These tiny countries at the southern part of Persian Gulf are artificial countries that were created by the Western bloc and their regimes are installed by the same powers so their very survival depends on their creators.
This is why we consider them US colonies and it is also why they have no choice but to remain in a cycle of selling oil <> buying useless US weapons. Unlike what you said, they aren't buying it for their "defense" the regimes are basically paying a high tax to ensure their existence.
Take the Patriot defense system. Each missile costs between 3 to 6 million dollar and they shoot dozens of them to bring down an aircraft worth a couple of grand and they can't do that. The aircraft comes and drops its bombs and goes back to its base while they waste tens of millions. Then they go ahead and buy more of it despite knowing how useless it is. Why? Simply because if they stop paying that "tax" the regime that was installed by US will be replaced by another one.