[mixmax]

Another alternative is that a nimble startup creates a piece of software to solve the problem and sells it to 1000 companies at $10.000 a pop. This way the companies get their software even cheaper, and another beautiful startup is born.

With a contrived example you can prove anything :-)

[DarkShikari]

Of course, but most of the time when software is created for an internal use, the company doesn't want to be in the business of selling said product.

[rubinelli]

Most custom-built software is too tied to internal processes to be useful outside, because a generic solution always costs more (the consensus is 3x). Most successful OS projects that have this source solve a narrow, highly technical problem.