> 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.

No you don't, because WASM is only compute, and you need exactly runtime specific code and third party frameworks for everything else as imported functions.