That's a reason to use it, but it's not the only reason.
For example, where I live it's much easier to recruit JS developers with React and React Native experience than it is to recruit native mobile developers (it's not a cost thing; there just aren't many native mobile devs looking for work). Consequently it's better to build with React Native even if you're only targeting one platform because the long term support for the app would be basically impossible if you can't recruit someone capable of doing it.