That doesn't turn it into a physical reality like a stone or a stream of water that exists regardless of what animals think about it. Territories exist because they are defended. They are not obvious unless one deals with the means employed to defend it.

The need to defend might be a necessity for survival, but the desire to defend additional territory and resources has existed ever since humans have acquired the power to achieve more than the means of mere survival. Similar to food preferences, which become peculiar if there is plentitude, basic if tight, and sub-par in emergencies: during famines, sometimes people resort to eat grass to sate their feeling of hunger even though digesting it is an energetic net negative.