Adding cores to a CPU is not quite as trivial as all that. You have to consider the implications of shared memory and caches and the overhead of bus traffic bet

Adding cores to a CPU is not quite as trivial as all that. You have to consider the implications of shared memory and caches and the overhead of bus traffic between the cores. As the number of cores increases the overhead begins to dominate the design rather than the cores themselves.... there's been lots of good research on how to overcome those issues but it's very much an open problem at this point.