Because there is no competition. If you decide to build rocket launchers and the company you source, say the warheads for the rockets, starts sending you lemons

Because there is no competition. If you decide to build rocket launchers and the company you source, say the warheads for the rockets, starts sending you lemons (rockets without the high yield explosives you used to get for example), you start sourcing your warheads from someone else. Hell build you own and get on with it. Now if a company patents a specific warhead that only they can make and you decide to make a rocket launcher that is made specifically to launch rockets with those warheads and no other warheads are compatible with your launcher and suddenly the source company decides that their warheads suck anyways, you just lost your business. My Takarov can fire any type of 7.62×25mm Takarov ammunition, albiet some is better than others. If it could only fire a specific type of surplus ammo that might not longer be available tomorrow, my gun could end up being useless in a blink of an eye.