>if one of them wins, the other by definition must lose.
No. If you sell shares to pay for living expenses you do not lose. If you dilute your shares to raise capital to make your remaining shares worth more you do not lose.
The analog to that goes something like: all participants in a hand of blackjack can win. And even the house doesn't ”lose”: it makes an expected payout, which incentivizes the players to continue in the game, which over time will enrich the casino more.