OK, let's consider what happens inside a power supply. Firstly, the mains passes through a bridge rectifier in which short bursts of high current pulse into a big electrolytic capacitor. That's the kind of capacitor T6 is always warning us about, but a big one. This gives you a capacitor with approximately 1.4 x your nominal mains voltage across it. Then a small feed from this is taken to a transistorised oscillator (probably in a chip) which pulses a switching transistor on and off a few tens of thousands of times per second. This pulses current from that big capacitor through a coil of wire (the chopper transformer primary), inducing current in a second coil called (you've guessed it) the secondary. This has multiple taps which are in the right places to deliver the voltages in the right ratio for the computer. One of these is measured and the pulses to the chopper transistor are adjusted to set the output to the right voltage. The pulses from the secondary are rectified, smoothed with more of those nasty capacitors, and fed out to the various parts of the computer. Every time the load varies (say a disc spins up or down or you plug something into a USB port) the voltages dip and the pulses have to be adjusted again to return them to the right value.
Now the problem is that interrupting current through a big coil generates very big voltage spikes. We're talking about thousands of volts, and these are lethal to transistors, so the spikes have to be controlled with beefy networks of diodes and resistors to protect the transistor from these spikes while still ensuring the switching generates the right shape pulses to feed the correct output voltages. That took decades to get right, as it's not easy. It only takes one big pulse to destroy that transistor, under all the changes of pulse timing which might be required of it to keep those voltages right. And then there's the heat. That transistor along with everything else in the PSU gets hot, so hot it has to be cooled with a fan, which sucks in dust which clogs the airways and makes the cooling efficiency fall with time, and hot components change value, so all those carefully calculated protective networks start to change their characteristics and might no longer offer that transistor full protection. And I haven't mentioned the spikes. Cables can and do get struck by lightning. Cities need different amounts of power at different times of day, so power stations have to be switched on and off to supply the right amount, and that all causes spikes - momentary pulses of double or treble the normal voltage which only last a thousandth of a second or so, but that's a long time when you're switching on and off 20,000 times a second or more. One of those spikes adding to the normal pulses inside the PSU and it's curtains for that transistor.
The marvel isn't that power supplies occasionally fail. It's that they last as long as they do.
