The airlines don't want you bringing and consuming your own alcohol on their planes because: They earn money by selling their alcohol to passengers. The stewardesses are able to control how much alcohol the passengers imbibe.

FAA regulation 14 CFR § 121.575 strictly prohibits passengers from consuming any alcoholic beverage on an aircraft unless it has been served to them by the airline. The primary reason is safety and control: flight attendants are trained to monitor passenger behavior and must be able to cut off someone who is becoming overly intoxicated. Alcohol affects the body more quickly at high altitudes due to lower oxygen levels and cabin pressure, which can lead to unpredictable behavior or "air rage." If a passenger self-serves from "mini bottles" or a flask, the crew has no way of knowing how much they've actually consumed, which creates a liability and a safety risk for everyone on board. Violating this rule can result in massive fines—sometimes exceeding $40,000—and can even lead to being banned from the airline or facing criminal charges upon landing.