Heat.
While the actual LEDs (*) may be rated up to 20,000 hours+ in optimum circumstances, but the actual 3rd party bulb manufacturers, especially the cheapo brands, are building bulbs with poor heat dissipation designs and cheap and/or poorly designed circuits. Same goes for other parts they may use, such as the power supply. To reach 20,000+ hours, you need everything - not just the LED - to be working optimally together.
(*) the best LED makers out there right now, e.g. Nichia, Cree, Phillips - really do some amazing engineering.
Never. Never is the correct answer. He's been delusional his whole life.