It would take a massive amount of electricity for a company to use even a small percentage of the amount if hardware they could sell.

They can sell money printers for more than they will print in their life. And as someone said once you have the money upfront from selling the hardware you can make/sell another batch while the customers are left slowly hashing away.

Also nobody is saying that they don't use their own hardware because they all most likely do. Just that selling hardware is much more profitable than mining alone.