Because engineering a functioning car from a bunch of junk is hard. I would guess most of these require 1.5-2.0x shop-time-to-road-time to keep them running. Once that stops being fun and starts being a pain, they get rolled out back and parked.

Also, depending on the (geographical) state they're in and the title situation, it may be difficult to impossible to get once licensed for actual road use. If all you can do is bang it around the back 40 or trailer it to places for off-road use, I'd guess the novelty eventually wears off and, again, the get parked up.