Web developers are a dime a dozen - very low barrier to entry, both from learning standpoint, and quality standpoint (sadly). C++ developers - a lot less. C++ developers cost more. Native developers who are also willing to learn C++ are also expensive. My cynical view is that with RN, startups are just able to hire a few web developers instead of having to hire native developers, so it's a lot cheaper.
>Bonus: you can test your business logic without having to run it on a phone.
You can do that with JS (or TypeScript etc.) as well. Even Java (or Kotlin) and Swift, or anything really, if you care enough to separate it from the UI parts.