The problem is that generally people who have several drinks lose their common sense, and you can’t determine who will drink more and who will stop.
My father was an airline pilot, and often recounted stories of passengers who would become unruly and create safety concerns. He often used the expression “There are no road shoulders in the sky”, meaning that if there are problems you can’t simply pull over.
Also, most everybody who is drinking at an airport bar is boarding a flight, so if the drinker is drinking too much, they are going to bring the consequences of that over drinking on board.