There was also an implicit saving back then that an error message could be looked up in some other system (typically, a printed manual). You didn't need to write 200 chars to the screen if you could display something much shorter, like SYS-3175, and be confident that the user could look that up in the manual and understand what they're being told and what to do about it.

IBM were experts at this, right up to the OS/2 days. And as machines got more powerful, it was easy to put in code to display the extra text by a lookup in a separate file/resource. Plus it made internationalization very easy.