The world is not an object anyone can reach. A street is. So is a ward, a classroom, a warehouse floor, one family with a difficult history.
People who actually change conditions tend to have worked at that size for a long time — the same street for thirty years, knowing which flat has the broken boiler and whose son is in trouble. It's an unimpressive answer to give at a party and it's the only scale at which anything has ever been repaired. Healing is retail work, done in small units, mostly by people nobody has heard of.