I recently did a lot of wiring in my old house and was always thinking about why there are separate neutral and ground wires when they both connect to the same terminal in main panel. Why can't it be only one wire serving both as ground and neutral? more precisel, why neutral contact of all 110 V fixtures couldn't connect just to the ground wire?

Hi Everyone, I am planning to locate 4, 50 watt outdoor rated halogen lights at the 4 corners of my driveway. I will be using hard PVC and outdoor rated 14/2 cable. I understand the requirements for he hook-up from the electrical panel to the exterior junction box that will be affixed to my exterior wall; however, I need some assistance in determining the best way to effeciently route my wire to these 4 lights. I would prefer to have the conduit (and wire) come down into the ground, directly at the side of the house where the junction box is affixed. From there, I would like to run a single conduit approximately 6 feet (barried underground) to a barried junction box where two conduit lines emerge - one line would travel from the junction down the Left side of the driveway and the other conduit line would run accross the driveways and then bend at 90 degrees and run dow the right side. Can I burry a waterproof junction box in the ground or do I need to rethink my plan so that I run 4 seperate conduit lines from the junction box above ground that is affixed to the house? I will appreciate your thoughts on this matter