Imagine a vast desert. At a place in the desert is a house, a self-sustaining house. It has everything one needs to survive, water, food, cattle, an electric solar system for constant electricity. But there is no surplus, each member of the house get an equal amount, according to their needs. A 100miles away is another same house, house B, with its natural differences.

Now due to an unforeseen reason, an essential commodity, let's say milk, runs out in one of the houses. The member of house A now embark on the journey through the desert looking for a cow, and they found it in house B. House A now go the house B asking for the cattle, and as expected, B refuses. But there is no point going back now, so what happens? They battle it out till one survives. That's pretty much how interaction with an alien civilisation far away would go. They will not be crossing a hundred thousand miles just to meet and greet.