It's because despite being far more efficient than incandescent they are still only about 30% efficient at producing visible light. The rest is heat, and unlike incandescent an LED does not want to be hot. The device must be cooled or it becomes less efficient and wears out faster. There is no good way to get the heat out of the front of the device because that's the side you are supposed to see, so in practice all the heat is removed from the back, i.e. the part inside the fixture.
Other solutions to this include using much larger devices, but that costs proportionally more and has application issues because people want their light bulbs to act like either line or point sources, not as areal sources. So most lights on the market use a single small LED, unless they are targeted to a buyer demanding high efficiency and long life, like a city streetlight.
