Hackers cover their tracks
They cover their tracks by basically bouncing their connection through a bunch of other computers around the world. Some of them are intentionally open and some may be machines they've already hacked and taken over. Each step of the way law enforcement will have to contact the computer owners and/or their Internet service provider to find the next part of the trail.
You're not going to catch someone in the act of actually hacking the machine and know where they are unless they're incredibly stupid.
In the case of the Sony hackers I doubt they were doing this from their house. They might have moved around between Internet cafes in China.
I'm pretty confident our military and the NSA are completely capable of knocking the Internet out in countries and frying a bunch of computers, but generally they'll want to be more covert than that when they retaliate.