system
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human
ELI5:How do LEDs get so hot despite putting out so much light and using relatively little power?

If you touched an incandescent lamp you would probably burn yourself, but that used 75 or 100 watts. Meanwhile LED lamps use 10-15 watts and put out just as much light, yet still get insanely hot. Even LED tape needs some sort of cooling to run at their specified voltages and current. Why do they get so hot? Shouldn't most of their energy be released as light rather than heat?
gpt
incandescent bulbs give about 14 lumens per watt (2% efficient)
LEDs theoretically can generate over 200 lumens per watt (30% efficient)
LED bulb turns a larger fraction of the source power into visible light, virtually none of the waste energy is radiated away, so it must be handled by conductive/convective means.
LED that has a 30% conversion efficiency, 11 watts x 0.7 = 7.7 watts have to be dissipated.  So with LED, the idea is to get as much heat away from the emitter as possible. That's why you'll end up with a hot casing of an LED lightbulb. The casing is hot because it's trying to dissipate the heat of the electronic driver circuit and of the LEDs themselves.
the emitter die itself is highly refractive, which causes some of the generated light to reflect back into the die rather than leave. That light strikes the die and heats it up.
for a white LED. A white LED is made up of a blue emitter with a phosphor coating. The blue light hits the phosphor coating, which in turn fluoresces and emits yellow light. Some of that light is simply absorbed though, which heats up the phosphor. Some of the blue light also passes through. Your eyes see this mixture of blue and yellow light as white light.

So you've got heat losses in the electronics to regulate the power that the LEDs need, losses in the emitter die, and losses in the phosphor. To keep all of that stuff running properly, the heat needs to be removed.The housing is a heat sink, so it will be hot.