Sesto Elemento is a variant of the Gallardo.
For one, the Gallardo is not even in the game. Second, it's not a variant of the Gallardo. It takes the engine of a Gallardo Superleggera (which we never had in a GT game), and makes a completely different car that reportedly weighs less than a ton. They don't share anything in the exterior with one another, which is, from a modeling stand point, the most important factor. The interior is also completely different. Comparing this, to a rebadged GT86, or the Silvias, or whatever, is absurd.
Italdesign Zerouno is a variant of the Huracan.
Yes, it's based on the Huracan, yet it uses a completely different bodywork. So, unlike the duplicate GT86 and Silvias and such, which were added just as a quick way to boost the car count without spending any resources by copying an existing model, while getting the Zerouno would actually take a lot more effort, since they would have to model the car entirely from scratch. Sure, you could that there should be other priorities, and I agree, over the Zerouno, but still, is not comparable to the duplicates in game.
Maserati MC12 is a variant of the Enzo.
Once again, you are comparing two cars that share their "innards" but have a completely different bodywork, brand and even performance, with cars that are literaly the same, with a different badge, or with different wheel options, or whatever.
Yet people complain about the variants of the Skyline, Silvia, and Miata, all of which have different appearances and performance specs compared to the "base" model...some to greater degrees than others. These listed cars are just as much of variant of their respective base car as the S14 kouki and zenki.
Nope, they aren't. It's not even a debate. And once again, comparing two trim levels between the same exact Silvia*, with cars that look (and in some cases, perform) completely different, from different brands, is just delusional.
But don't worry, PD won't bother adding these "duplicates" as they are not japanese cars, in favor of actual duplicates made in Japan.
*reminder that you can get engine swaps in this game. So certain cars, could literaly get any engine the brand has/had available for that model, without PD needing to duplicate the car itself. Meanwhile, you can't just "makeshift" a MC12 out of a Ferrari Enzo.