Because it would make it slower, less reliable, and well overall shittier at everything.   
See sig, I'm not a fan of auto's but the DCT/DSG boxes of today are amazing. 
I presume you know they are not an "automatic" but merely dual clutch/split transmission rig that has no torque converter?   
Shifts faster than any human could ever. 
No loss of boost between shifts, while not having a torque converter = no loss of acceleration during shifts.     
The only true "loss" is you are not doing it manually. 
I can't think of anyone who would really want a GTR in a manual.