From my experience, I find most power supplies use bare minimum rated components such as capacitors.
They'll go with 85 degrees C and voltage ratings just under double the working voltage. While 85 degrees C is quite hot, the closer to this temperature, the shorter lifespan of the capacitor. When I replace capacitors, I always go with 125 degrees C and higher voltages (providing they physically fit).
Also, power supplies are so tightly packed that the design requires a good balance between component location and dissipating heat to the casing.
If power supplies were designed better, they'd last longer, but the price will increase. A computer power supply can be designed without a fan, but the heat sinks would be quite large (unless the transistors were mounted to the computer tower case). Unfortunately we live in a throw a way society. Years ago you bought a TV that lasted twenty to thirty years. Now the TV goes bad in five-years, the repair price is 75% of the cost, and you discover the new TVs are more modern thus making it not worth repairing an old unit.