There may be problems in the implementation details depending on the university, but the general idea of TAs is a good one, for two primary reasons.
One, in many fields, the more advanced you are, the harder it becomes to remember or even understand why or how one may struggle with understanding the basic material of the field. TAs are still closer to the experience of having learned the basics themselves.
Two, teaching material is a great way to deepen one's understanding of the material.
It also happens to provide a nice balance of giving students a way to earn some money while working in the subject, while still being quite cheap for the university. (That last part does have a smell especially in US universities with their insanely high tuition fees. But that feels like a larger and largely separate issue.)
They already do. They’re called TAs and grad students.