They have lead, etc. In them. If not properly recycled (they won't be) that'll end up in the environment.
And they cover up and mar the beauty of the land. Maybe that's fine if you live next to a desert.
There are lead backing sheets being used with PV, but they aren't required, and are being phased out in Europe and some other places. The lead is incompatible with some PV new higher efficiency silicon PV technologies due to processing temperature, so there's additional reason to get rid of it. Bifacial PV cells would not have a backing sheet. In any case it is not a requirement for even monofacial PV.
