Take a look at Raytheon's manufacturing line: https://www.youtube.com/watch?v=kCCkVAHSzrc That's what it takes to have missiles that are nearly guaranteed to perform to specification every time. You can stockpile the packaged missiles in a non-climate-controlled shed for years, replenish them at sea while being showered with salt water, subject them to shock of a nearby blast while in a VLS, and they will still launch, go up to Mach 13, and catch an incoming ballistic missile nearly every time.
Sure, Iran's ballistic missiles are simpler than SM-3, but they are still subject to most of the constraints. They still need perfectly cast large size solid rocket motors that don't crack after being stored for a year, they need warheads that only go off when they are supposed to, they still need to trace every part for QA, etc. There's a vast gap, largely invisible to amateurs, between garage prototypes and stockpiled AURs.