Clouds are made of water droplets and dust in the atmosphere at altitudes of 1 to 16 km. While travelling through air, due to friction, the water droplets get electrically charged.
Lightning occurs due to sparking between oppositely charged clouds – a high voltage spark rushes towards the ground (at zero potential) through moisture-filled air.
If any around based structure is on the path of the spark, the top of that structure is excited to a high electrical potential while its bottom (in contact with the around) remains at zero voltage. This high potential difference sets a very high current in the structure causing sudden heat generation in the material and destruction. But in flying aero-planes, there is nothing like a ground point which remains always at zero voltage.
Hence even if hit a lightning, the plane’s entire surface acquires the same high potential and due to lack of potential difference there is no current and hence no destruction.