Barriers to entry. First would be capital. And given the current market, supply chain is probably even more important than the knowledge of how to make it.
If the big companies could produce more at these higher prices with similar costs, I guarantee they certainly would be. To an extent anyway since they would be dumb to completely flood the market so price drops too fast. But as far as economics, it's much easier for them to make more than a new player to enter the market. Even if they need more land and buildings, they have supply chain, permits, etc. Likely why no one has done it, the barriers are serious and the risk is high, they would get wind of it and could easily crush a new, smaller player in the market.
It's not like they cut production way back to raise the price. Sure, maybe covid is effecting output a bit but demand went up way more than supply went down. Every ammo company I know has added shifts and employees to capitalize on the market, not the opposite.
Not trying to rain on your parade, I have big dreams too and follow them and even make them happen when everyone says it's not possible, but you asked and that's my businessman answer lol