That's because they are cars on the line-up. But really, why is this? Because they share parts with other Nissan models? I think if you include the 280ZX and 300ZX (Z31 and Z32) as Z cars, then you have to accept the Z33 and Z34. The Z31 shares many parts with many other vehicles of the time. Purists will tell you that anything after the 280Z (or 240Z!) are not Z cars. Less purist people will tell you that the S30, Z32 and up are the only Z cars and that the S130 and Z31 were "mistakes".
It varies a lot depending on who you talk to, but they were all sold as Z cars. I'm not sure what makes something like the Z31 elusive, especially since the Z31 sold the most in the states, had the worst suspension and handling, and was quite lethargic compared to the other series.
ZR8ED has quite a lot of experience with Zs, I believe. Also, your inflation figures are a little exaggerated. And the Z32 was closer to 40k back in the 90s, so 60k in today's money. I think the Z33/34 feel like Zs, it's just that they are new Zs, so you don't have that suspense back in your mind where you are wondering if somethings going to break. What would be really, really, cool would be to swap a VQ in to it, like a couple other people on this board has. The VQ has proven itself quite the successor of the VG.
That's a little condescending; care to explain what's so mysteriously complicated about it? Did you read about how ZR8ED already started, completed, drove for 10 years, and sold a VG30ET S30?