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.
It's not just a matter of magic, but what sort of magic. There are hundreds of magical beings across their world, after all, in LA and other places. Twilight can obviously pinpoint which signals are the Slayers.
Even if that wasn't the case, there's no indication that even Willow could generate enough magic equal to the amount of a whole Slayer army.
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!
It was last resort, and they did make it as heavily fortified as was within their abilities. They don't have all that Chinese military artillery just for kicks.
Because if you're looking to find something or someone, you *always* start in their last known location.
Their last known location was somewhere in the middle of the ocean, not somewhere in the mountains of Tibet.