I have 2008 VW Rabbit with just over 40,000 miles on it. Why did my rear brakes go so quickly?

I would not accept that it is a defect. If you have done 40 k miles then you should have had a brake inspection during your service schedule. The brakes work in a very hostile environment and the inspection would have identified whether there was any brake "drag" due to the effects of salt, road grime or corrosion caused by road water. To expect a brake to work for 40 k miles without being stripped and re-embled with new pads is expecting too much.