Let me start from the basic level that will be from roots of making Corks.
Cork literally traces its roots to a special type of Mediterranean oak called Quercus suber L.
Harvesting cork does not kill and certainly does not involve cutting down the cork tree; the life of these valuable trees is actually carefully guarded by law.
Cork—a type of outer bark called cambium—is peeled off of the tree in thick layers.
The tree is then left to grow a new cork layer which can be harvested again in about nine years.
Each tree can be harvested for about 200 years with little or no recourse to fertilizers, herbicides, or irrigation
So, the raw material for Corks is literally free and it doesn’t harm nature.
Screwcaps are made from aluminium plus a plastic (or sometimes tin) lining to prevent the aluminium from contacting the wine.
Knowing that aluminium could make contact with the fluids and might degrade its quality and plastic being non-biodegradable; Corks are preferred over them.