Well because the 370z isn't the car a lot of people want. Its way way more expensive in convertible trim. Its also a very heavy car. If you were to pick a competitor to the miata I'd say something like the BRZ is way closer. (no vert I know) I should ask why you're so dead set against the idea of a performance variant of the Miata? A 200ish hp variant around $30k would probably do pretty well.
You
do realize a "performance variant" of the Miata was already tried, correct? The Mazdaspeed Miata.
And the sales were horrible.
There was also this car called the Honda S2000. Basically, everything you're asking for. It's sales slowed to almost nil in the waning years of its life, despite selling for WELL under invoice and having its own niche within a niche.
As, frankly, most "niche" variants of already niche cars tend to be. The Solstice/Sky GXP/Redline were the "performance variants" of the two last true Miata competitors, and they sold far below that of the NA versions of those cars (and, ironically, less than the MX-5)
There is the internet forum/car magazine/paper racer idea that these types of performance variants on extremely niche cars need to exist for bragging rights and the "true enthusiasts", but then inevitably, they rarely put their money where their mouth is and actually
buy the cars. They're great for arguing on internet forums and busting out e-Peen 0-60 comparisons, but too few actually show up at the show rooms.
Companies like Mazda aren't McLaren, or Ferrari, or even Porsche. Making sub-categories of already niche products that don't sell well already for "bragging rights" is not a recipe for success.
Also, the latest iteration of the MX-5, the NC3 "Club", clocks in around
6.0seconds 0-60 (6.1 stated by Motortend).
That's not OMGBBQWTF "fast", but its definitely respectable. Certainly when factoring in how if you're buying a Miata to go fast in straight lines, you're doing it wrong.