They had to have free time to carve the stones. That means either localized hunting, fishing and gathering was very very good, or they had some sort of localized agriculture.
I agree. I’ve been a hunter for 30 years, and I know that even in game-rich spots, it’s an uncertain activity. I can’t imagine that a group of people who lived by hunting could muster enough spare time to build a megalithic complex. There must have been some sort of agriculture involved.