I dunno, are you sure you can manually write correct de/serializaiton for JSON and XML so strings, floats and integer formats correctly get parsed between JavaScript, Java, Python, Go, Rust, C++ and any other languages?

Do you want to maintain that and debug that? Do you want to do all of that without help of a compiler enforcing the schema and failing compiles/CI when someone accidentally changes the schema?

Because you get all of that with protobuf if you use them appropriately.

You can of course build all of this yourself... and maybe it'll even be as efficient, performant and supported. Maybe.