Why are companies selling mining hardware instead of using it themselves? I'm not (yet) using Bitcoin, and I'm just approaching this world as of now.

I stumbled upon some merchants that sell mining hardware (with some GH/s). But I've yet to understand what they gain, apart from the upfront cash.

The reason one should buy mining hardware is, if i'm not mistaken, to "generate bitcoins", i.e. "generate money"; so, one expects to be able to have a return of the investment sometime in the future, so I guess that in some way my claim is correct (about "generating money").

But then, why don't the companies simply use their own hardware?
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.