like david mentioned, it only costs money on another money note, I work for the large electric utility here in so cal and almost every year we are asked by various consumer advocate's groups to bury our large 220 to 500kv lines (the big lines that are on transmission towers) under the soil. they argue that this will eliminate fires caused by down power lines. Yes, while it may substantially minimize the occurence, it costs substantially more to bury lines that large into the earth. oh, and the "rate payers" (thats you and me) flip the bill and that usually stops the discussion right then and there---to a point.

maintenance becomes a huge issue as you will need to excavate those lines if need be. its the exact reason and parrells the reason why many undeveloped countries almost always prefer to set up wireless networks vs wired underground or above ground (overhead) phone lines.