This is indeed ray tracing's huge advantage, of being highly amenable to parallelization. However, the same thing is more or less equivalently true for other rendering methods, such as rasterization. Modern GPUs have thousands of rendering sub-unit "cores". And with CUDA you can use those cores for almost anything, including ray tracing. But for a given scene and a given amount of finite computing power you're almost always going to be able to get superior results using a rasterization technique than ray tracing.