As a former bartender and occasional lone drinker the explanation I can offer is sometimes one likes the feeling of being alone but surrounded by social activity.
On times when I have been living dirt cheap in single rooms, it’s nice to just get out.
Not having like a living or tv room, even if you wouldn’t do anything other than stare at the TV next to other people, one can find yourself craving just a bit of other people in the surroundings.
Personally, especially feeling drained after work it could give me a bit of a feeling of recharge, self reflection and peace.
Also it can be nice if you randomly bump into someone you know or strike up a conversation with another lone drinker.
Of course that can be risky as they can obviously turn out to be a problem, but then other times can turn into hilarious fun unplanned nights!