Because a LED bulb with a heat sink will emit many time more light then the screen of a iPhone. A low power LED bulb do not need a heat sink.
Just try to illuminate a wall in a dark room with the screen and compare that to a light bulb.
There might be a difference in efficiency and the phone product less heat for the same amount of light and that is because how you it is constructed so you can run a LEDs on mains voltage versus the battery in a cellphone. A LED bulb often is made as cheap as possible but the cellphone charger and voltage converter in the phone is more advances, expensive and efficent. But the main difference it still the amount of light they emit.
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