They ARE reusable - up to a point.
Depending on the type of coating on the composite layers it is possible to remove the gasket without disturbing the coating and reuse it. But my experience (on V8s) has been that after a couple of remove/installs the coating will delaminate and further use is risky.
So, just as you can reuse head bolts a couple of times before they start to take a permanent plastic set, you may also reuse the head gasket a couple of times, assuming all the laminations survive the previous head removal process.
