Before we go on, be reminded that reliability is a complex probabilistic science. Yes, probability and statistics can be tricky, but they are necessary for determining when a computer is likely to fail. This does not mean that yours or mine will last X hours. It only means that ours will probably last about that long. There is no way that a manufacturer can determine exactly how long any given computer will last, just as it is not possible to determine exactly how long you will live, assuming normal circumstances.
Manufacturing defects tend to be the single largest cause of computer hardware failure.
Manufacturers cannot guarantee that every device they make is perfect. However, some are better at minimizing the number of problems that occur. Computer, for instance, does an amazing amount of testing on all of its computers and peripherals, as well as keeping close track of failure rates in the field. Most well-known manufacturers do a good job in terms of design and manufacturing. You will nearly always be better off, in terms of reliability, if you buy equipment from reputable manufacturers.