And there is nothing that says that it should be accessible. Which is exactly why a terminal, going by the very definition you yourself provide, is very difficult to work with from an accessibility perspective. There are simply too many degrees of freedom, allowing you to represent data in too many different ways, making it very difficult for something like a screen reader to "parse" the buffer (which can contain anything, not just text) and convert it to something meaningful.

Using a terminal, one can implement a progress bar in hundreds of different ways - and it would be impossible for a screen reader to handle all such use-cases.