Maybe that is true, yeah. They may not have the motivation to be a singer / performer / artist, or they've tried and failed and need to make money.
I kinda think of it as...well, singing teachers either want to be teachers but don't want a heap of kids around all the time or strict school working hours, and they happen to be able to sing, so they teach that. Or they WERE singers / performers in some small sense, but then get old, can't sustain the lifestyle (not successful enough to afford it), so they become teachers as their day job.