It's trivial to do. People have developed programs that just generate absolutely huge numbers of bitcoin addresses using your GPU.
Why don't you build a powerful network and just run these for a few months, checking each created address to see if it has any bitcoins in it, and let me know how many bitcoins you steal.
This makes no sense. You can generate the addresses on any GPU or asic you want, and chance that you generate any address that has ever been in the block-chain is close to 0.
List of addresses that have any BTC is even smaller, something around 150MB. If checking generated address against those addresses would have any chance of success bitcoin would be dead long time ago.