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Because hackers get into their servers, steal all users info and sell it quickly. It happened to Sony's Playstation and Valves Steam this past year or so. Target was recently hit for those that had Target cards I think. No server is really safe from hackers.
Or you basically
ask a low-wage customer service rep to give you access, or at least gather as much information as you can. (If you ask the right kind of manipulative questions, of course.)
All the encryption in the world doesn't matter if the front line defenses are have direct access to the sensitive information, are poorly trained, and probably don't really give a damn about keeping your data secure. They probably care more about a supervisor breathing down their neck if they spend too much time on the phone with each customer.
Just like warehouse work, or a UPS/Fedex distribution center: They don't care about getting packages onto the trucks intact. Are the packages on the truck they need to be on? If yes, then good. If some are damaged, oh well, at least they're on the truck. Now it can be someone else's problem.