The TET's level of technology was clearly far superior to that possessed by Earth at the time of its attack. Even if a comprehensive plan of attack could have been mounted before the TET destroyed (and thus human civilization) there's simply no telling if that attack would have been successful. And after the Moon was destroyed, any human resistance would have been disorganized and more geared to combating the "Jack" clones sent to Earth , rather than attacking the vessel which sent them.
Basically, the TET's arrival was probably not seen as being a threat until it was too late to organize a coordinated defense against it.
