It is based upon very old unix and VMS. In the early 80s when unix (i.e. Sun Solaris) machines were first coming on line, there were no standards as to how or where to mount your drives other than the root one. So you needed something that was easy to type (i.e. short), identified which physical drive it was (for when it died). With oracle, each mounted drive was used for one datafile which was never accessed by anything other than Oracle. Early versions of Oracle had limitations on the length of the paths for their data files. Plus you (the dba) needed to use command line tools and edlin to fix configuration files so Oracle could find it's data files after a drive died.
To sum up, for Oracle 6 and 7, you wanted short, easy to remember drive paths which mapped to physical drives for quickness of maintenance.
Since that time, standards have been created, modified, management tools added. But that documentation has never been changed. Because it works as described. And smart DBAs can figure out how it works in their systems. And if they can't Oracle Consulting (at $2K per day per head) will be more than happy to do it for you.