There is no law that requires it, I find most banjo players did start on the guitar but not all-I have played banjo for 5 years and don't know a single guitar chord.
Like a lot of adults it's just a hobby for me, I have a job and a family and don't have much time to play or to practice. I only have time for one instrument and I happen to like banjo better. Now if you wanted to join a group, professionally or semi professinally then you might want to learn rhythm guitar so that you can back up other players, and you will appreciate other players who extend the same courtesy. But in every informal jam I've ever been in, there were plenty of guitar players. I have seen a few teachers insist on guitar first then banjo but in my experience they just happened to play the banjo as a sideline and really teach better on the guitar. No organ teacher will take on a student who hasn't played piano, you need keyboard training for that, but most good banjo teachers will teach you banjo even if they happen to be great guitarists.

There is no law that requires it, I find most banjo players did start on the guitar but not all-I have played banjo for 5 years and don't know a single guitar chord.
Like a lot of adults it's just a hobby for me, I have a job and a family and don't have much time to play or to practice. I only have time for one instrument and I happen to like banjo better. Now if you wanted to join a group, professionally or semi professionally then you might want to learn rhythm guitar so that you can back up other players, and you will appreciate other players who extend the same courtesy. And it is fun to know more than one instrument. But in every informal jam I've ever been in, there were plenty of guitar players. I have seen a few teachers insist on guitar first then banjo but in my experience they play the banjo as a sideline and really play and teach better on the guitar. If you want to play a complicated church or theater organ then few if any teachers will work with you unless you already play piano-that's reasonable since the organ has so many voices and options it is a waste of time to learn keyboard on it. But I don't think any good banjo teacher would ever insist on guitar first, they have different sounds and chords and generally players and students have reasons for wanting one or the other.