If you bought the shares after IPO, then someone sold them to you, and they missed out on all of those gains. Even in the IPO, it's very common that you're buying shares from insiders and private investors rather than from the company itself.

(But even if trading is zero-sum in a strict monetary sense, different risk preferences, desire to liquidate positions in order to buy something you value more, etc. mean that it's not zero-sum in a utility sense.)