Mostly because no one ever thought of it.
Parafin wax is used in motor oil because of it's non stick properties.
Thermo-reactive wax is used in thermostats in cars due to it's ability to mold to imperfections in steel casting.
Soft candle wax is used to make jewelery setting molds.
As for wax being able to get the detail of fingerprints, pour some wax out of a candle, wait for it to cool a bit, and press your finger in it.
Shall I continue?
The reason no one has heard about using earplug wax for casting is probably because no one thought about it. I've got a couple friends that are chemists, physicists, and just all around sciencey, and they all seem to agree with me. I'm actually going to head to Walmart to get some ear plugs later today to try it.