You are unconvinced until it hits you. I never heard about it before it started bothering me.

It's individual. I have a friend, overweight, never played any sport, never saw him run, sits 16h a day, sleeps 8h a day, has no problem with his back. Another one, skinny, does sports, sits on a pilates ball at work, uses standing table, stretching, and still has back problems.

You can't make conclusions based on your own case. We are all different people, different physiology. Medicine is not maths, 2 + 2 is not always 4. The fact that they can't explain RSI doesn't mean it doesn't exist. It's just that we don't fully understand what's going on there (yet).