I will give you a possible answer born of long years of experience. Many people - far more than will be admitted - are actually quite nervous flyers. A drink can help them deal with this. Another reason is that a great number of people are stressed through work or life in general - they relax with a drink. Then there is society as a whole and in Northern Europe and the British people, it is now part of the culture.

This does not excuse drunkenness. I have on one occasion confiscated 14 bottles of alcohol that people had bought in Duty Free and which were doing the rounds. It was ridiculous as it was all free on BA at that time before this BOB nonsense started.

I cannot answer for public transport in general but only on aircraft in particular