GPU stands for Graphics Processing Unit while CPU stands for Central Processing Unit. For CPUs which has say four cores, we at max can run four threads simultaneously (one thread on each core). In graphics(e.g. image) processing, so many pixels are processed simultaneously. For so much simultaneous processing, we need so many threads running simultaneously. But as I told before, for a CPU(which has limited number of cores), we can run limited number of threads simultaneously. Hence we need GPU which consisted of many more cores, specially in a case of graphics processing(e.g. Games on Computers).
For your information, Nvidia's GPU GeForce GTX TITAN X consisted of 3072 cores. For it's specification, go through http://www.geforce.com/hardware/desktop-gpus/geforce-gtx-titan-x/specifications.