OK Gertrude. Since you want to blame everyone except the drunk himself:
1, Drunk on boarding. Would have been spotted by cabin crew
2. Alcohol supplied by cabin crew. How much can you drink in that short time?
3. Alcohol carried through security. No way.
4. Alcohol purchased duty free. Yep, and probably poured into a soft-drink bottle to get past the cabin crew. Thinks he's (she's) clever. Throw the book at them.