Correct me if I'm wrong but I believe GM has to do a full emissions, safety, crash certification process for every engine/car combo they produce. Which is seriously time-consuming and expensive. It's not like the old days where you could order the combo you wanted and they could build it (wish it was like that still). Dropping an LS7 into a 1LE just doesn't make economic sense when you consider they're already at the end of the 5th gen run and are gearing up for the 6th gen.
It works to make a new model like the Z/28 which sets the wheels-in-motion (no pun intended) for this category of car in the 6th gen. That puts in place the development team, the department, the budget and all the necessary corporate mechanisms to have this car become dominant in racing. Production cars that are dominant in racing sell street cars.
I find it HIGHLY interesting that Dodge has already declared they're working on a direct competitor to the Z/28. You KNOW that Ford isn't just sitting there watching all this and doing nothing. Do I detect the first breezes of a new manufacturer's championship series in the near future? I don't think the timing of all this is a coincidence. We are in very interesting times for auto enthusiasts.