Because if I'm honest, I do know them. I've been working for several years at the same place, have made friends, had lunch/dinner/beers/coffees, been to weddings, etc. Some of them are not friends, but I am happy to see them, just like I enjoy talking to my butcher or fruit seller.

Maybe it's an age/cultural thing, but I'm going to spend quite a bit of time working, so I'd rather do it with people I like. Inevitably, because we like each other, we're happy to see each other, hence the office is a good place to go to. And if I don't know them yet, there's no reason why I shouldn't meet anyone interesting at the office.

I also enjoy talking about my work with like minded people : not my friends, but co-workers. The best conversations happen over beers, coffees, meals, etc which require physical interactions.