[Yetanfou]

That's because 'help' on most Linux-based systems is actually a shell builtin which explains other shell builtin commands. This, in turn, is related to the fact that Linux-based systems are flexible - you could use a totally different shell which does not even have the _help_ builtin - while VMS was a comprehensive single-sourced top-down controlled system. Both have their strengths and weaknesses, the fact that you are nearly certain to be using several Linux instances right at this moment gives some indication of which of these systems eventually turned out to be the strongest, warts and all.

[PaulDavisThe1st]

help on VMS wasn't a builtin. It was a system-provided program, with data files that could be accessed via other software.

[JdeBP]

The same was true on MS-DOS, DR-DOS, and OS/2. Microsoft provided a help file compiler with some of its DOS development tools, and IBM provided an INF/HLP compiler and (if memory serves correctly) a message file compiler with the OS/2 SDK (HELP using both hypertext help files and message files on OS/2).