So are a lot of single-player games. Players running around in the background just add some semblance of "dynamic life" to the world. There's also the possibility of "well, I'm not playing with people right now but I could if I really wanted to".

I'm not saying that your perspective is wrong on that (I tend to bounce between the two views for example) but for some people just being in the same world as other players provides enough dynamism to make it feel more interesting than playing completely on your own.