AR-10s are heavy. The bolt itself is much larger and way heavier. The receivers are substantially larger and the buffer needs to be heavier to compensate for the much larger round. The barrels need to be larger as well. Then the weight difference of a loaded .308 mag vs a .556 mag. It adds up quick.

POF makes a badass AR-15 in .308 that is ridiculously light. Unfortunately the Revolutions are damn near impossible to find right now.

AR-10s are heavy. The bolt itself is much larger and way heavier. The receivers are substantially larger and the buffer needs to be heavier to compensate for the much larger round. The barrels need to be larger as well. Then the weight difference of a loaded .308 mag vs a .556 mag. It adds up quick.