Collisions, congestion, and gridlock
Single-robot obstacle avoidance assumes the world outside the robot is either static or moves in ways the robot can just react to. In a warehouse aisle shared by dozens of robots, that assumption breaks down: every other robot is also planning, also reacting, and also trying to get somewhere. If two robots each independently decide to swerve around the other in the same direction, they can end up right back in conflict. If enough robots converge on a narrow aisle or intersection at once, none of them can move at all, a traffic jam that a single-robot planner has no concept of because it was never designed to reason about other planners.
This is why fleet systems layer traffic management on top of individual path planning. Intersections are often treated like reservations: a robot claims a time slot to cross a junction, and other robots are held back or rerouted until that slot is free, similar in spirit to air traffic control rather than a single car's lane-keeping. Aisles that are too narrow for two robots to pass can be treated as one-directional at a time, with a coordination layer deciding who goes first based on priority and what's queued behind them.