I presume the reason they do it is that the premise of Nushell is that it uses pipes of structured output instead of simple text streams. That means that they need all the tools to output datas in that form. They could include wrappers for all OS provided binaries and handle the conversion in those wrappers, but that makes you incredibly fragile to minor output or flags changes, and in many cases those wrappers would end up being more complex than the complex than the commands themselves.