If you have 10% chance to lose -$91 and 90% chance to get +$10, the expected value in money is -$0.1. It sounds bad.

But the relationship between utility and money isn't linear. If for you, $10 is worth 10u and $91 is worth only 89u, this deal has expected value of +0.1u.

Why and how isn't it linear? It's a hard problem that can't be answered easily. However we know it's true for most big institutions in stock and bond markets. A financial product will need to provide extra expected value in term of money to compensate the risk, otherwise no one buys it.