But, Katy59, as has already been pointed out. If someone gets really tanked up before they come into the airport, they will still be drunk when they get on board the plane- and who is to stop people coming into an airport building from bringing alcoholic drink with them and drinking it either openly or secretively? or is everyone coming into the building, which will include all people meeting people, saying goodbye, meeters and greeters, staff etc going to be breathalysed and body and luggage searched as they come into the building?
More generally alcohol consumption is falling and while drunken incidents on planes are unpleasant, they are relatively infrequent.
There are strict rules governing consumption of alcohol by air crews and as long as those remain in place, drunken passengers are a nuisance, bvut they are not flying the plane and cannot get access into the cockpit.