"A question that archaeologists seem to me to be reluctant to tackle is why archaeological remains have value to humanity...."

Maybe because the answer is so obvious? Archaeological remains are the physical manifestation of past culture. Who could doubt that past culture is of value to humanity?

The value to humanity of Stonehenge in World Heritage Convention terms is immense, as high as you can get. The current argument is whether that applies to the Landscape too. EH et al are effectively saying it doesn't.