It's as silly as saying that buying health insurance causes you to get sick. A prenup makes it less likely that you will get divorced because it strengthens the relationship going in because you've made a lot of hard decisions and have had a lot of dialogue about tough issues. A will is not a substitute for a prenup. You say you're living with a partner. I don't know whether it's a living situation or marriage situation.
Most states provide for automatic spousal benefits upon death.
In addition, a will is a one-person document that can be changed at any time without the knowledge and consent of the other person. A prenup is a two-sided agreement that cannot be change without mutual consent. But I prefer to think of the prenup as having prevented a divorce. That question kind of reminds me of the famous Seinfeld episode where George wants to get out of his engagement and is advised to ask for a prenup in order to do it. George Costanza was advised that the best way to get the wedding called off was to ask for a prenup.