I couldn't tell you exactly why Jamf doesn't do it. My theory is that Apple doesn't want to let the ability to connect and view another iPad screen out of their control. And Jamf wanted to create something with more overall control than Classroom so they made a product that works in conjunction with Classroom. This is, of course, is only a theory and I have no actual knowledge of any reasoning behind it.
That said, I also wish that Jamf Teacher had the ability to observe student screens. Trying to get teachers to use one app for classroom monitoring is hard enough, then you try to have them use another is very difficult. I also have to toss in the complaint about Classroom connectivity/connection that constantly has issues dropping students or not allowing them into a class that they were in just the day before. But I digress.