The map is a snapshot, the world is not
Even a well-built map is a snapshot of the environment at the time it was captured. A warehouse AMR's map doesn't know a forklift will be parked in an aisle tomorrow, and a delivery robot's map of a sidewalk doesn't account for the specific pedestrians crossing its path right now. Static maps are necessary for global planning, but real environments constantly deviate from them in ways the robot has to detect and react to on the fly.
This is where local planning and dynamic obstacle handling take over from the global plan. As the robot moves, it continuously scans its immediate surroundings with LiDAR or cameras, comparing what it currently senses against what the static map predicted. A mismatch, an object where the map says there should be free space, triggers a local replanning response distinct from the long-horizon global route.