Because there is one huge problem - the power usage of the CPUs / GPUs / ASICs. If you buy a "mining rig", probably an ASIC costing $1200 - $6000 (you can spen

Because there is one huge problem - the power usage of the CPUs / GPUs / ASICs. If you buy a "mining rig", probably an ASIC costing $1200 - $6000 (you can spend as much as you want scaling up and across) up front and it consumes something like 800 watts of power 24/7, you have the up-front cost of hardware plus the power bill (800W continuous is 584kWh per 30 days which depending on your area might be like $62 per month per 800W rig). You may never recover those costs, especially if newer more powerful ASIC rigs are release each year since more overall network computing power diminishes your existing rig's efficiency. Also - hardware breaks down under maxed out continuous usage, so there's that too.