At the limits of their ratings. They could make LED bulbs last many orders of magnitude longer and be more efficient, but they don't (unless forced to[1]) because they prefer planned obsolescence.

[1] https://news.ycombinator.com/item?id=27093793

Placed under operating conditions very close to their specified limits.

Like if you were to drive your car in 2nd gear on the freeway, at 6000rpm. The engine would wear out much quicker than if you drove in 5th gear, at 1500rpm.

100 years of training make most people think of light bulbs as a trivial purchase. And now a product that cost $0.50 20 years ago is $10, and often performs worse for its purpose.

So the economics just drive cost down no matter what. And even a picky consumer is hard pressed to get what he wants when you go to the bulb aisle at Lowe’s. They literally went from 10 SKUs to 250, with no meaningful standards.

In most cases, they are run too hot with barely adequate heat dissipation.