Out of universe answer
In very early drafts of the script to the original Star Wars, <PERSON> used the phrase "May the Force of Others be with you". This was intended to be a common greeting, a reference to a shared religion that was common in many inhibitants of the galaxy he was just starting to build ideas about at the time. 42
In the original rough draft of the script, the phrase is used by several characters: both the King and Queen of "Aquilae" (a world which seems to have become merged with Alderaan during later revisions, as they are <PERSON>'s parents), "General" <PERSON> (an elderly Jedi), <PERSON> (his old friend) and <PERSON> (<PERSON>'s son), <PERSON> (an anthropologist...) and <PERSON> (a random rebel with only 3 other lines).
It was this phrase that inspired him to develop the ideas about what the "Force of Others" actually was (shortening the name to just "the Force" along the way, probably just because it sounds better like that -- <PERSON> is very driven by the way things sound in his writing), and what the Jedi (or Jedi-Bendu as they were called in early drafts) actually did.
Along the way, he changed his ideas about the background of the galaxy, and followers of the Force became rare, almost extinct (although it doesn't seem he had yet figured out exactly why: I think he was just trying to highlight <PERSON> character as being exotic, and explain why <PERSON> had never encountered the ideas behind the Force before), but I suspect he simply didn't have the will to completely remove that line from the script -- I suspect he loved the sound of it, and it had a personal link for him to the history of how he'd developed the story -- so he left it in, even though it wasn't entirely logical for it to be used in that context, at least according to the ideas he was developing at the time.
Of course, later justifications were added, but as of the moment the final Star Wars script was written, it didn't make much sense. But <PERSON> didn't expect Star Wars to be the commercial success it was, he wasn't expecting to develop it into a full 6 movie series, and he certainly wasn't expecting the huge extended universe or other additions that came later. So he let a slightly illogical line stand, because he wanted it to be there.
(This is, of course, just a theory: <PERSON>'s statements about the actual process by which Star Wars was developed are self contradictory and in places nonsensical, so a lot of this is based on supposition, but it seems reasonable) 42