I start a company to make cheap autonomous flying cars. I invest $1,000 of my own money and build a working prototype. It looks promising, so you buy 50% of my shares for $10,000.

I just made a 20x return on my investment, and you own 50% of a great opportunity. Who's the loser here that makes this a zero-sum market?

Our autonomous flying cars go into production, and now your 50% is worth $100 billion, and you sell your shares to Elon Musk. Where's the zero sum?