The semiconductor of LEDs, or light-emitting diodes, is made up of a negatively charged layer with free floating electrons and a positively charged layer with openings for the electrons. When the electrons are activated by an electric current they flow into the holes of the positive layer and produce light. LEDs become less efficient over time and lose brightness.
Part of the problem is the very newness of LED lamps. Traditional lights have well established methods of testing life hours that are industry-standards. For example, the life hours of incandescent bulbs are tested by burning a statistically significant number of sample bulbs continuously. The amount of time until half of those bulbs burn out is considered their life rating.
Measuring the life hours of LED lamps is more complicated. It still involves having a sample group lit continuously, but not until they burn out. LED lamps don't go out all at once like other bulbs, their glow slowly fades. LED life hours are a measure of the amount of time until the lamps are down to 70% of their original brightness.
The 70% of original brightness standard is used because research shows that the human eye typically can't tell that a light is decreasing until it has dimmed by over 30%. The newness of LED lights again becomes problematic because many performance projections are based on extrapolated data instead of on real world testing.
Another problem with the claimed 25,000 and 50,000 lifespans is that they don't take failure modes or component malfunctions into account. Even if the lights could last for 25,000 to 50,000 hours under perfect conditions, the lamps have real world vulnerabilities that can cut their life hours short.
Environmental factors have a huge effect on the lifespan of a LED light. Heat is said to be the top cause of LED failures. If a light's heat sink is inadequate or not installed properly the light will overheat and stop working. This heat sensitivity also applies to the temperature of the room the light is in, and high ambient temperatures can cause overheating. Other environmental factors include humidity and moisture.
LED lamps and their fittings are more complicated than incandescent bulbs. More parts means higher chances of malfunctions. LED lights are vulnerable to fluctuations in voltage and current, component failure, damage to connections, and phosphor degradation.
In my opinion, warranties are the best indicators of how long manufacturers really expect their products to last and LED lamp warranties average 3 to 5 years.