A closed source project can license closed source technology and save development costs while avoiding putting 1.0 code for an edge case into their core product. The exchange of money helps keep the interest in support, maintenance, and extension by those who first developed the technology.
Open source projects can also release v1.0 with bugs, it's not like there is some Open Source Police stopping you from doing that. You can also pay for support of open source software, and that benefits everyone and not just the company that happened to have money to invest in it.
There's still money licensing Fortran libraries that have been around for decades. They are fast and good and cheaper than achieving the same levels of fast and good from scratch. There's nothing intrinsically wrong with using some gem written over a weekend of craft beer. It just may not make sense to bet a business on it even when you can read the source code
StackOverflow is an example of why the choice is beneficial. The source code being proprietary is irrelevant because the content is Creative Commons.
