Bare copper wire;

Why do we use bare copper wire for grounding, why not all insulated?

Ever seen a green wire, that's a ground too.

Bare is almost always used for bonding or making surfaces equipotential. If you got a lightning strike and the strike went to ground, the insulation would be toast. Uninsulated wire is less expensive to make.

Romex such as 14/2 with ground, still has somewhat of an insulated sheath because of the outer covering. Lightning strikes destroys this covering easily.

Insulated wire also will do of same gauge. But why to pay for more cost of insulation? Hence, bare conductor is used. There is no harm if insulated conductor is used.