Accessibility is a big one. When your buttons are actual UIButtons, they get treated as such by the iOS accessibility system.
Most developers don't consider accessibility at all because they don't need it themselves. But there are millions of iOS and Android users who do need it, and they might be thankful that you've made their life easier with your app.
Following platform standards makes things easier for users. Users don't care if your app has distinct graphics.
Sorry, I don't know how React Native does it, although my understanding is that Button components do end up as real platform buttons.
Generally speaking, UIButtons have important semantics that are easy to miss if one just reproduces the appearance using custom graphics. The same applies to other standard controls too.
Most developers don't consider accessibility at all because they don't need it themselves. But there are millions of iOS and Android users who do need it, and they might be thankful that you've made their life easier with your app.
Following platform standards makes things easier for users. Users don't care if your app has distinct graphics.