That isn't true.That’s a popular comment during shortages/panics but it just doesn’t bear scrutiny. An entire reloading industry with powder manufacturers, bullet manufacturers, brass manufacturers, equipment manufacturers wouldn’t exist if primers were only incidentally available.
As an example, Vista is reputed to be 20 million behind in ammo contracts. They are running their ammo plants full tilt and use all the components that they produce. Occasionally run into shortages of materials...copper, brass, lead...and have to stop production. When that happens the components that don't need to missing material continue in production. That creates an overrun of that component, which is sold on the commercial market through their distributors. When their supply lines get caught up and they can produce more cartridges, the supply of components on the open market dries up