Why there?

Ever notice how people tend to congregate in the narrowest part of a "walkway"? Two people can be walking through a completely empty 20 x 20 room, but will stop in the doorway to carry on a conversation. Or you can have a hallway that has a filecabinet/desk/etc. in one spot, reducing it to a single-file width . . . and that is where people will gather to talk. Why? Why do these people pick the absolute most obstructive areas to congregate in? Then, when someone (usually me) has to get past them, they'll move the absolute minimum amount of distance they possibly can. Just barely enough to be noticible, but not nearly enough to get by. And if you ask them to move some more, you'll get a glare like you just told them to jump off a pier. What's so difficult about taking an extra 3 steps (or 3 fewer) before carrying on your pointless conversation in a less restricted part of the hallway/room/area instead of blocking everyone else's path?