Maybe its due to the perceived build cost for a new car, which is often quoted as being in the mid 30s for a 40. Therefore, why would you buy a s/h older car for the money you could buy & build a new one for. Unfortunately this overlooks the time to build, or the cost to have someone build it for you...
I know not all builds are equal too.
Its a shame as some cars are trully outstanding, but still don't make their true worth imo.