Because It wouldn't feel as an upgrade. With arrows and bullets you use them as usual - buy, shoot. With endless things you are getting an upgrade, which removes "buy" step. But since you will be able to get that right after first rocket weapon, It won't be an upgade, and Rockets I will become useless immedialty. Musket balls and Arrows is still useful, since you still can turn them into better ammo.
So, his and my point - there would be no point in having rockets 1 as a thing, we could just make cyborg sell endless rocket canistre instead.
My personal opinion - we shouldn't be able to craft it, but we should be able to buy it from ceyborg after, lets say, Martian invasion.