I believe the answer is: They did not build towns or cities.
There are 3 basic criteria for a civilization:
- It must has a permanent place
- It must have people as members of the civilizations
- It must has some sort of government that regulate the people
Hunter gatherers move from one place to another in order to find foods and other resources needed by the group. This mean that they do not have permanent place to settle down. Because of this, hunter gatherers cannot be considered as a civilization.