Not all text editing occurs on 80x25 screens. In fact, there are text editors for screens that are 2x25, 40x25, or even 132x25.
A text editor often has to make smart use of the screen real-estate if it is to work effectively in smaller format. Line editors have a natural edge in the small formats.
Windowing systems have reduced the impact of having small screen sizes. However, scrollbars are not the solution to all display problems. Sometimes, you just need an editor which can "get out of the way" when you're composing.