"Maybe we are not looking at the entire house here", Andrey's thought cut through the room like a lightning creating a silence our discussion was not used to. We were, the 3 of us, re-calculating everything we now knew about the dolmens. This simple thought had changed every thing. It is not a house, it is part of the house. The rest of the house might be constructed of cloth, animal skins, ropes and wood. The dolmen is like the wear-house maybe or the fridge of the house. The people would be semi-nomadic, having 2 or 3 "camps. One up north for the long summer days and one south for the winters, for example. When they leave, everything that they leave goes into the dolmen. It makes sense for the dolmen to be a sealed chamber, it allows the people to leave grains and other dry foods in it, trusting that it will not get wet nor eaten by animals. When the people come back they construct their shelter around the dolmen again.
It makes a lot of sense to put a lot of effort into building something robust, durable and sealed if you leave it full of goodies for a few months while you travel north to the other camp.