Usually excessive heat and current. There is no incentive for the manufacturer to make them last a long time, so they use fewer LED dies and put more current through them. The result is less efficient, and the LEDs die faster. They save a couple cents on BOM cost, and you have to replace the bulb more often. Both are a good thing, from the manufacturer’s perspective.
Putting the bulb in an enclosed light fixture just makes the problem worse, as those trap heat.
There are a [bunch of possibilities](https://en.wikipedia.org/wiki/List_of_LED_failure_modes). The failures could be related to the packaging, to the LED semiconductor chip itself, or the conditions under which it was used. To know the exact answer for a particular LED, of course, would require failure analysis of that unit.
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