Clouds taken from plane

This is a picture taken from the plane on the way from London (UK) to Los Angeles (USA).

When climbing up to an altitude of approximately 12 km, these beautiful clouds formations can be observed.

Airliners fly at flight level (FL) from 9 km to 13 km. Why do they fly at this altitude? The air is thinner and therefore less resistive (fewer molecules resist the aircraft), less fuel consumption = flights are cheaper, less turbulence and generally less negative weather conditions, more comfortable flight for passengers, air traffic is better planned (scattered to many flight levels), pilots will have more time to deal with in crisis situations.