Why? Open source coding is essentially pro-bono work, that's required by professional ethics in many fields. If we want to be regarded as valued professionals, there are some obligations to the general public that come with that.

For that matter, most developers are not being paid to work on the likes of VLC or some other COTS, where "should this be open source" is even a meaningful question. They're developing or maintaining some internal enterprise app, so any source code they develop will always be available to the final user in that basic sense.