What is the reason that some departments do not allow PhD students to be instructors on record?

It is common that PhD students have teaching duties as TA. In this case, the person ultimately for the course (the instructor on record) is a faculty member. I should note that I am talking only about enrolled PhD students at the same university as where the undergraduate course is taught.

In some places, it seems that PhD students can serve as the instructor on record, but at many other places they are not.

What are the reasons why PhD students are not permitted to be instructors of record at some universities in the USA and Europe?