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.
Back in the (late) 90s, for $10 or so, Elwood would record a customized "you've got mail!" message for you. I had him create one for my (now) wife, using her real first name, as well as her nickname. "Elizabeth, you've got mail!"