By untyped I assume you mean dynamic languages? I’m some contexts it’s not convenient to lug around a type checker, embedded languages for example. Other times if doing macro heavy programming (lisp, forth) it’s hard to build a type system that can properly type check the code or resolve the implicit types in a reasonable amount of time.

In the context of JSON you can work on it without types from a typed language. It’s just that as a force of habit coders may chose to spend time adding types to things when they shouldn’t.