Little Nismo is right in his original, if long winded, statement.
It's a road car. A multi performance road car. The GT3 is required to spend as much time as possible at or above 100mph. Lack of mass causes instability as the speed increases. Down force gives you grip, but not much in stabilising the car at high speed. That's why racing drivers operate at a different level to all of we wanna be racers. Mass is required for stability. Mizuno San was tasked with creating a stable and fast platform, not just fast. That's easy.
I'm all gobby tonight, and I'm not allowed to drink. Antibiotic soberness sucks.