Most luggage locks aren't to keep thieves out. Luggage locks are an inconvenience, not a preventative. If someone wants to steal your luggage, they'll take the whole thing someplace private and just slice a hole in it.
Luggage locks are to prevent casual pilfering, including from sticky-fingered luggage handlers who aren't part of the TSA inspection crew. They also serve to help make sure that the bags stay zipped closed during potential mis-handling, so the contents aren't spilled out if a zipper gets caught.