Beer has 0 fat, 2.6g carbs and 1g protein...they why does it makes you fat?

That's Michelob Ultra as given on the regular 12 fl oz bottle. Its light beer alright, but I have been having 3 or 5 12-fl oz bottles every weekend (which is not much since I dont drink any alcohol on weekdays) since the past 2 years, and I have been steadily putting on weight. If beer has 0 fat and only 3gs of carbs, then why do people put on weight drinking it?