Because if Twilight can follow a signal created by their magic, and obstructing that signal screws up magic, duplicating it would be the next logical step. Willow activating hundreds of potentials without screwing much magic, so doing something with the appearance of that shouldn't be a major issue.
Standing still while functionally disarming should be a tactic of absolute last resort and in a heavily fortified position. Not in a wide open area with no cover!
Because if you're looking to find something or someone, you *always* start in their last known location.