Why ceramic IC packaging?
In IC packages it certainly is desirable to dissipate heat with the lowest possible thermal resistance.
However, at the same time, electrical insulation and protection from oxidation / corrosion are also desirable, at least for discrete components that are likely to be handled or exposed to the environment.
An insulating packaging such as ceramic or plastic allows this insulation and protection, while permitting heat dissipation through controlled paths, such as integrated heat sinks or heat sink tabs in some packages, or just through the pins in others.
Many IC packages are also sold as bare die, or wafer level chip scale (WLCSP) packages, for the circuit assembly process to directly connect to the PCB. The bare chip is then environmentally protected using epoxy potting or similar protection coatings, after soldering or bonding of the lead bumps to the circuit board.
Such bare packaging of course requires more sophisticated assembly equipment than the much larger IC (and larger contact pitch) packages do, so they aren't for everyone.
The type of chips most commonly seen in ceramic packages are those with UV-erasable memory. In order to allow such memory to be reused after it is programmed, it must be possible to expose the die to a considerable quantity of UV light. This requires that the chip have a quartz window, and installing a quartz window on a chip in turn requires that the chip's package be made of something whose thermal expansion characteristics reasonably match those of quartz. If a quartz window were installed in an epoxy package, thermal expansion and contraction of the package would likely cause the seals to fail, allowing atmospheric air (including water vapor) to reach the part and destroy it. I saw one chip once which looked like it was made from epoxy with a plastic window which looked a bit "milky"; I didn't examine it closely enough to confirm that, though. If it was a plastic window, it would probably have been usable for a few UV-erase cycles, but many plastics degrade relatively quickly UV exposure. Perhaps someone figured that making EPROM chips with plastic cases would save enough cost that even if they would fail after a few uses, they'd be reusable enough to justify using them instead of non-windowed parts, and cheap enough to justify using them instead of ceramic parts. I don't think they ever caught on, though.
The main other place I've seen ceramic parts was in places where they had a metal top which would be heat-sinked. There again, the dimensional stability of the ceramic was necessary to prevent the seal from failing under changing temperature conditions.