Why are companies selling mining hardware instead of using it themselves?
Companies that manufacture mining equipment don't just mine with their own hardware for a variety of reasons:


They may not be able to obtain low prices on electricity, making it more profitable for others to mine than for them to mine.
They may not be able to make use of the heat generated, making it more profitable for others to mine than for them to mine.
It's easier to get financing if you're going to make something you can sell.
They may prefer not to take the risk that mining will remain profitable.
The value of mining equipment depends on the value of bitcoins, which would be negatively affected by mining becoming too consolidated. So mining is more profitable if it's distributed.
It may be virtually impossible to mine profitably and they may be too smart to think otherwise.