They're highly customized and they're cheap. Why get somebody else's dreams when you can buy your own? On top of that, the brand has little to no name recognition. Very few "name" players use Carvin. To go even further, as somebody who owns two Carvins I do not think they are god's gift to basses like some people do. Carvin makes a solid bass but some mediocre electronics which is a huge turn off for a lot of people. I am not that shocked that Carvin's re-sale value is so low. I am surprised by Ernie Ball Music Man and Warwick's low re-sale values though.
Yeah the electronics are garbage but the wood and the craftsmenship of the basses is amazing.. I have thought about getting a nice Carvin 5 string and putting EMG pickups and preamps in it.. as with any bass good electronics will bring out all the tone in even lower quality basses.