I think this is a legitimate question and I also find it very upsetting.
There are a few likely suspects, and all probably play a degree to one point or another.
1. Planned obsolescence. This is another way of saying "we'll make a special part and then stop making it. We know the part will generally last so long, and then people will be forced to buy a new car because we'll stomp all over any aftermarket producer, too."
2. CAFE standards. Generally, to get better gas mileage, you need a car that weighs less. In order to make a car that weighs less, its (often mission-critical) components must weigh less. The lighter construction of items like electronic assemblies and switch-gear don't allow them to last as long as older, heavier comparable items. This is especially prevalent in items like alternators, power window switches, and so forth.
3. Cost engineering. We need a part. The pencil-pushing pinheads in accounting will only let us spend X dollars on this, and they must cost no more than X dollars per unit. You get what you pay for.
4. Bad design. This has always been a problem. From my observation, it's become worse. Sealed units, non-rebuildable assemblies, and so forth contribute to short component life.
5. Heat. Today's smaller cars tend to get very, very hot under the hood. As a result, electrical components, which are sensitive to ambient temperature, don't last as long because of the hot temperatures. The light construction previously alluded to is also a factor, as there's less material to act as a heat-sink, or, in some cases, to help dissipate the heat.
6. Poor assembly quality. It still happens, even with lasers, robots and all manner of other autmatonic marvels. People are still involved, errors will happen.
7. Well-assembled junk. You can have perfect, statistically-consistent assembly, zero defects, and all the other slogans that would make Dr. Demming spin in his grave, but if you're still building with junk...guess what you get?
These are my opinions. I'm sure others have theirs.
Charlie Larkin