I remember reading a John Paul Jones interview in Bass Player magazine back in the 1970s and his answer to that question was pretty close to mine.
Some friends were forming a band. There were plenty of people who could sing, play guitar, or play keyboards. They needed a drummer and a bass player. I thought about drums. So much gear to lug around and set up and dismantle. All your arms and legs going at once, all doing different things. I thought about bass. A combo amp and one guitar. Only four strings. One finger on each hand. I chose bass. And that's a true story.
There is another bit to this story though, which is also true. When The Partridge Family was on TV, I was about 9 or 10. I distinctly remember watching it, not knowing at the time that they were just actors (except David Cassidy) and thinking: If that fat obnoxious ginger kid can play bass, so can I. So Danny Bonaduce has to take some credit...