The cheap ones (because that’s what I’ve always bought): Why does one ear always go out? If you jiggle the wire the bum ear kicks in and out. You can hold it in an awkward position and listen good. But move a bit, and the one ear crackles, goes silent, maybe kicks back in. Why? What’s going on here? I’d like to know from an electrical perspective.
A loose connection somewhere. Wiggling makes the broken connection physically complete, but any movement may move the ends apart.
Not necessarily the cheap ones. Years ago, I had a very expensive set do the same thing.
Although the standard 3.5mm stereo jack has three connectors (one signal for each side and a common ground, this divides into a pair of conductors for each side before it exits the plug (at least, it does that in the cheap ones where you can peel the two wires apart from each other as far down the flex as you like).
So in order for both to fail at the same time, something would have to go wrong with the plug itself. In order for only one to fail first, something could go wrong with anything that isn’t the plug.
And the reason you don’t find that one side fails, then the other fails a couple of weeks later is probably that you’ve already replaced them by that point.
The wires in those earphones is incredibly thin. You will lose the connection either at the ear or the jack on one or both sides. Pulling and bending break the wire quicker. Low quality can make them bad to start with. If you like micro surgery, you can slice the wire open lengthwise and connect the good sections. It’s really not worth it beyond an experiment.
This has happened to every set I’ve owned. The Skullcandy pair (not cheap) I bought a few years ago lasted the longest: They’re only now breaking down.