I've seen this first hand living with an alcoholic. The addiction is just so powerful that it's the only thing that they will crave. Even if they have some food here and there, it only takes one or two days of drinking to keep the cycle going. This is just one individual though, a lot of it probably has to do with one's metabolism and tolerance to alcohol.
Your body doesn't want to metabolize large amounts of alcohol for energy. It's just trying to get rid of the poison as quickly as possible when large amounts are consumed. It's not going to be a 100% conversion rate like sugar.
As someone raised in a family of alcoholics, where's the skinny? Most those who need to drink are obese.
If you're seeing people asking you for money then they clearly don't earn the money to feed their addiction, let alone feed themselves food to gain weight.