> you dont need to write platform specific code

You don't need to write platform-specific code if you use some cross-platform framework. For simple programs it may be enough to use only the standard library of your language of choice.

> single portable binary that can run on x86 windows, arm64 linux and in your browser with zero modification

It has little value. Compiling a separate binary for each OS isn't that hard, since only a handful of architectures and operating systems are actually in use. Using an abstract cross-platform binary (like WASM) in the other hand adds extra performance costs and other user-side overhead, which isn't strictly necessary.