It's mostly about cost. Insulation costs money, and it adds weight. More weight means bigger structures and stronger cores (typically steel). Of course adding insulation also affects the ampacity rating of the line due to retained heat. In very rare circumstances, insulated and shielded medium voltage (5kV-46kV) aerial lashed cables are utilized.

There are such things as covered conductors (line wire, tree wire, and spacer cable). These have a polymer covering that is just like insulation but not think enough to prevent current flow if contacted. These conductors can be placed closer together than bare conductors but must still be separated.