Mr. Rilch is an electrician, so he knows how to wrap the light strings so as not to find a box of “spaghetti” when we unpack the ornaments and such. He also knows how to repair the fuse on a burned-out string.
But what he doesn’t know is, how does a string of lights, which worked perfectly well the year before, burn out just from sitting in a box? You take 'em out, plug 'em in, and they don’t light up. They worked right up until the time you took the tree down, and now they don’t.
My off-the-cuff answer was “planned obsolescence”, but that still doesn’t explain what happens while the lights aren’t in use.
My WAG is that most strings of lights are stored in non-climate controlled environment such as garage or a storage shed from year to year. This puts a whole year’s worth of stress on the bulbs. The filaments inside the bulbs are probably not that durable and also probably grow brittle over time. Finally, strings of light get banged around pretty hard when they are being put up and when they are taken out the next year and uncoiled/untangled. For certain types of strings, all it takes is one broken light to kill the entire string.
Disclaimer: IANA electrician, so I don’t know how to avoid the dreaded Christmas Octopus Attack, but…
My assumption has always been that it’s only a $4.99 Made-in-China string of rather fragile electrical connections, and that it’s not sturdy enough to handle being flung about the living room the way you can fling an extension cord around with impunity. Face it, when one takes the lights down, one does tend to fling a bit.
And toss. And rattle. Rattle rattle rattle, there they go, into the box for next year. Along the way, I’ve always assumed, some tiny but crucial electrical connection breaks.
My lights are stored in a regular closet, so I don’t have the “what do you expect from an attic or garage” excuse.
WAG: Did you try just going down the strand, screwing/pushing the bulbs in a little tighter? I don’t know about christmas lights, but that definitely works for “burned out” regular bulbs sometimes. You think they’re burned out, but then it turns out they’re just slightly loose.