Wireless earbuds usually fail because their playback time collapses while the case still works normally. The cause is the size of the cells involved and how often they are cycled.
Cell size determines cycle frequency
An earbud contains a cell small enough to fit in something worn in the ear, which limits capacity to a few hours of playback at most.
The case holds a battery many times larger, because it only has to fit in a pocket. It therefore delivers several full earbud charges before needing one itself.
Battery ageing is driven largely by charge cycles, so the earbuds accumulate cycles at several times the rate of the case and reach the same wear point far sooner.
Small cells have less margin
Capacity loss is proportional, so a given percentage of degradation removes a similar fraction from both. On a two-hour runtime that fraction is noticeable immediately.
There is also less physical margin for the protective circuitry and electrolyte volume that help larger cells tolerate ageing, so the decline is less graceful.
The practical effect is that earbuds reach unusable runtime while the case, having lost a similar proportion, still holds plenty for its purpose.
The charging pattern is unfavourable
Returning earbuds to the case after every use means they are charged to full and held there, which is the state that ages lithium cells fastest.
Heat compounds it, since the cells sit next to charging circuitry in a sealed enclosure with essentially no ventilation.
Some products now delay the final portion of charging or stop short of full, which addresses this directly at a small cost in runtime.
Repair is generally impractical
Earbuds are assembled with adhesive around waterproofing seals and contain no removable fasteners, so opening one usually destroys it.
The cells themselves are welded to the circuit board rather than connected, which places replacement beyond ordinary repair capability.
Design constraints made it this way
Every millimetre of internal volume competes between the driver, antenna, sensors, microphones and battery, and the battery is the component most easily reduced.
Waterproofing and a seamless exterior both argue against any opening, and both are features buyers value more than serviceability at the point of purchase.
The result is a product whose lifespan is set at the design stage by a cell that will be cycled several times daily and cannot be changed.