Options trading is not zero sum because the utility of money is not linear. When a rich person gives money to a poor person, the net gain is positive because the poor person can use the money to satisfy more basic needs that the rich person has already satisfied, and thus would otherwise have spent the money on something with a smaller return.
As brentr already pointed out, some people trade options for risky gains (such as selling an uncovered call), and some people trade them to offset risk (such as buying an underwater put). Acting in the latter category is like buying insurance: even though your expected return on money is negative, your expected return on utility might still be positive. Acting in the former category is like selling insurance: you run the risk of taking a big hit, but your expected return is still positive.
As brentr already pointed out, some people trade options for risky gains (such as selling an uncovered call), and some people trade them to offset risk (such as buying an underwater put). Acting in the latter category is like buying insurance: even though your expected return on money is negative, your expected return on utility might still be positive. Acting in the former category is like selling insurance: you run the risk of taking a big hit, but your expected return is still positive.