The native build environment is tricky for a few reasons. On iOS it takes a slow download of Xcode and simulator, plus an Apple Developer account. Before CRNA, there was no open-source way to get start building an iOS app on Windows or Linux. As for Android, there are a long list of native build dependencies and environment variables that need to be correctly established before you can build your app.

It is definitely possible to create a cross-platform C++ library to share performant across platforms, but unfortunately you need to do the plumbing for each platform by hand.