It's not. You can look at other nations where they have a more diverse political landscape. What ends up happening is coalition governments where you might end up having 3-4 parties joining together just to stop a bigger party. Currently, in the US, the Republicans would be the biggest "natural" party. The democrats as a party in the US already exist in a "coalition" of sorts, it's why they say they are the "big tent" party. It's why there are conservative democrats, progressive democrats, single issue democrats. But if you look at the republican party, it is almost exclusively Trump/Maga conservative. If we fractured the democratic vote even more, Republicans would win, constantly.