My company develops software for internal use and we've even been asked about selling it by some customers who have seen it. We don't sell it because doing so would require a ton of work to decouple it from our other internal services/processes and generalize it enough to maybe be actually useable by other enterprises, would give us a whole second set of customers in totally different industries from that of our main product, and would generally be a total pain in the ass. I suspect that there's a lot more of that sort of thing than the other.