It's because of the currents. Have a look at any body of water with a bit of debris - e.g. a backyard pool with leaves on it. It tends to concentrate in a few areas.

It's a good thing in a way; it allows us to spend a lot less clearing it up.

It kind of makes perfect sense that it would all collect somewhere. I presume you've seen a pond or other still-ish body of water where scum accumulates in one spot.