You ask why astronauts cannot drink alcohol - I suggest to you that they have drunk alcohol. Cognac was certainly aboard MIR.
Whether they should drink alcohol is a better question.
There is, of course, no possibility of using extra-solar sources for beverages, especially those in a diffuse molecular cloud. I think that you have not grasped the distance to, and nature of, such clouds.
Jmaes, true. you can drink but its dangerous risk because alcohol is not buoyant in weightless environment became foamy mess during space travel. Rest you have explained correctly.