system
Given the following post, respond as if you're commenting on Reddit:
human
[Marvel/MCU] Why is the arc reactor a secret?

Howard Stark (with help from Ivan Vanko) figured out how to make the arc reactor practical. Tony Stark recreated most of it *in a cave* ***with a BOX OF SCRAPS,*** and later perfected it after discovering Howard's secret ingredient.

Now (circa Age of Ultron), half of New York is powered by the new giant arc reactor at Avengers Tower. By why stop there?

Based on some napkin calculations I've seen done, the miniature arc reactor Tony uses is as powerful as a small thermonuclear reactor found on modern nuclear submarines. It's self-sustaining for decades unless it is overdrawn, and if mass produced it would end the energy crisis the world over.

And yet, it's exact design is kept a secret and it is not mass produced. Why?
gpt
You can't just drop a new technology like that into mass production.  It's got to work its way through field testing to determine its optimal configuration from a technical perspective for large-scale energy production, plus any safety considerations. 

 There's the legal side with patents and the inevitable lawsuits that will come about from existing power companies being cut out of the market.  The economic ramifications of turning a core industry on its head can't be dismissed lightly.

And then there's the marketing and attracting investors.  Stark might be filthy rich, but no man is an island.  Even he is answerable to stock holders and his board.  The rollout of this technology is going to make a lot of people very rich and it must be done on a measured way.  Companies with the necessary production facilities are quietly being acquired, engineers are being trained, distribution networks are being primed.  

But rest assured it is coming.  You can't keep a genie like that on a bottle.  MCU Earth is in for some massive societal changes in the coming years.