Because while farming you can also complete patrol missions, bounties, and public events. Can hit plenty of daily PvE activities at once instead of playing the same repetitive PvP maps seeing nothing but Suros, and shotguns/fusion rifles. I play PvP when I need to for my exotic bounties, during IB, etc. but I would much rather turn in materials for crucible marks than grind out matches just to get gear that I would break down for materials.