You're missing that people write mountains of code in Python and similar languages; they are not always just for a small amount of glue to gain access to some specialized libraries.
Well, I assume people write mountains of code in the library. If you're making a machine learning product, that is still a lot of work.
However, writing your own Tensorflow interface would take several human lifetimes to get it right, and Google already has provided it. So it seems that is not the part you would re-write no matter how good the language is.
