Image Source
Graphics Cards are known for their profitability and you can add more than 10 gpus at same motherboard but you can use max 2 cpus on same motherboard. Even most expensive processors makes less than 0.5 cents with mining Monero so it takes more than 2-3 years to get your money back on cpu mining unless you have free electricity or solar mining farm. It can be profitable if you spec mine new released coins but most new released mineable coins have less liquidity so it's hard to sell them. Even at 10% less efficiency (off the top of my head number from something I read many many months ago) of "CPU > GPU", per setup and per how feasible it is to expand, GPU miners can grow rigs faster, easier, and more economical (in the long run) that CPUs can
That people don't do it is generally because we tend to chase profits and XMR just doesn't carry the price or the want to not dump and collect later that other coins do.
Much of that is due to the same issue we suffer from and that's the fact that not enough people have woken up to the fact that privacy matters
