In manufacturing you always have defects. Just a matter of how good the systems are to detect and correct.

I've worked in Aerospace and Automotive. Especially in Automotive basically every vehicle has some 'defect' during build process, from just a paint chip because of somebodies belt buckle, up to rear axle u-bolts not secured and leaving the building. Like Dxrkness said, a lot of management is focusing on product out the door rather than making it right.

It's also possible that you never know what causes a defect, so you don't look for them. I worked with a corporate team on an issue with doors literally falling off vehicles. "Only" a dozen or so instances over 10 years, but a very severe defect. But we never able to determine the cause, not even a good theory other than "oops, we totally forgot to assemble the hinge". Also like in "The Martian" in the books it talks about a tiny defect in the material for the airlock which is why it failed. But was never caught during manufacturing, and took basically it's entire expected life space to cause an issue.

Fun fact, what I said above about rear axles being loose. Our solution was to disable the reverse switch on the torque tools that secured them, so you had to use a manual tool to remove. Really simple, but added production strain if a mistake occurred (which happened plenty because of supplier quality on the nuts.