Because learning a new language with a different world model gets you to think about problems on a different way. A library for a language you already know will not do that to anything like the same degree.
Why? Libraries and grammar are both abstractions. The important part is how one can reframe problems and not how exactly it is written.
I suggest you actually try the exercise before making assertions like that. Mind you, there's the old saying, "Real programmers can write FORTRAN in any language"...