The same reason that filament based incandescent bulbs burned out. Planned obsolescence.

There’s a very real conspiracy (not just a theory) about the “arms race” in light bulbs for long lasting bulbs. Eventually, they made bulbs that lasted so long that they stopped making money.

Lighting manufacturers intentionally made worse bulbs to simply improve profits. They realized that they were driving themselves out of business. Everyone in the light bulb industry agreed to stop development of even longer lasting bulbs, just so they could continue to move units and make money.

Also, with LEDs, the thing that burns out fastest isn’t the LEDs (there’s usually a dozen… ish, in an LED bulb)… It’s the electronics. The power needs to be converted from line power to something the LEDs can handle, which is usually DC. So there’s a full power supply in the bulb to convert AC to DC, with a certain voltage to power the LEDs.

Sometimes this conversation is simple, a full bridge rectifier with little more than a filtering capacitor, other times it’s very complex.

The power supply in the bulb is usually what fails first.