It's important to remember that zero theft is a nice goal, it is not the optimum one. Apple could employ biosensors and release a toxic nerve agent whenever an unauthorized person tried to use the phone.
Walmart could have a security guard on every aisle, or assign one to every customer to follow them around the store. They could get a lot closer to zero shoplifting that way, but the cost (in wages for the guards, and lost sales from offended customers) might be higher than the cost of the shoplifting itself.
It's in Apple's interest to reduce theft, because customers who lose a phone before their contract is up may find the cost of an unsubsidized replacement to be higher than they are willing to pay, and they will look at other options.
It is not failure if Apple doesn't reach a goal of zero iPhone theft. Their goal is not zero theft at any cost. Their goal is significant reduction, at minimal cost.