I would guess that a lot of people who develop serious drinking problems end up drinking alone. I did. When I was out with a group, I was a very moderate drinker, nobody suspected anything. Now, I might have warmed up at home with a couple of drinks before we went out to dinner, or whatever, kept it very moderate when out, then once I got home, I did my serious drinking.
I think another reason people drink alone is that it is cheaper. Who can afford the price of "enough" drinks at a bar every night?