content: With miners, it's all about return on investment. They only need the GPU to mine, CPU + mobo just doubles the price without helping them make their money back. CPUs can mine cryptocurrency but it's a tiny fraction of how efficient a GPU is, so CPU's usually don't cover their own electricity bill let alone make the miner money back. And they don't need a mobo because they literally have 100s of GPU's hooked up to one (or just a few) boards.