>> that's where people choose to live.

Well, I would imagine the best policy would be to live <i>near</i> to the fertile land, not necessarily on it, but that's only looking at it from the point of view of large settlements. A small settlement wouldn't make too much difference I suppose in a very fertile area.

When you get to the Bronze Age though competition for land and resources increases, so why give it up then?

One odd instance is Beaghmore in Tyrone. The complex was built on to of an earlier field system, but after the land had just started to deteriorate. This is usually interpreted as an attempt to please the gods that (were obviously annoyed) by giving up more and more of the diminishing resource.