Ok imagine there is soft wipe. We are left with nude lvl 60 character. What can you do than? All of us would be stuck in neverending dead cycle: you dont have zeni cuz you cant grind-you cant grind cuz you dont have ANY items-you are forced to go to start location and grind there so its the same as strting from lvl 1.
Other thing is some items obtained as reward for lvling up through popo gift box and quests are pretty good like 4 CON necklace (normaly you cant get con stat in necklace), 100 resist rate necklace (for 60cap max resist rate in legend neckalce is 114), 5 free brown boxes and few others. Also going bit by bit lvling up you can start working on crafting skills as you loot and salvage items that are matching for your level.
Those are just few examples among many as for why starting lvl 60 is non-optimal