Why can't I ride a motorcycle when it thunders?

Because if thunder hits the ground and you ride a car, lightning will be transmitted from the ground to you. If you are wet, your own resistance will be small and lightning will hit you. So it's best not to ride a bike in thunderstorm weather.

The explanation for this is that,

The greater your span, the greater the potential difference, and the greater the possibility of being hit.

But in reality, this situation is still relatively rare.