The simplest reason why we wont get a Z.

ZCanadian

Other than the Vette and the 911, you can actually find practical use for those cars. Mustangs, Camaros and Challenger/Chargers come in many more flavours than "enthusiast". The higher ends don't really sell in huge numbers either.

911's sell for a different reason than 370Z's. But there are only so many dentists in the world.

The Vette sells because they take a good car and every few years make it an even better car (well, there were a few dog years in there, LOL). 25,000 copies moved of store shelves in the USA last year, and that's down for them from 34K in 2014. By comparison, the 370Z was 4,600 in 2017 and almost 7,400 in 2015. US 911 sales (all variants) were just shy of 9,000 units in the same period. Also down from 10,400 in 2014. In fact, sports car sales are down across the board.

Camaro - 68K down from 86K in 2014
Challenger/Charger 152K down from 160K in 2015
Mustang 81,800 vs 122,000 in 2015.

Corvette can share engine and suspension developments with other GM vehicles, and kind of is the sports car flag-bearer for the brand, which is probably how they keep it fresh.