The bigger question IMHO is why doesn't all public funded research happen in the open (or I'm so out of the field that I wouldn't know about it even if it happened).
I would love to donate some of my free time helping a project in which I believe with software engineering expertise. Heck I might even pick up some domain specific knowledge.
Here's one of the main reasons: competition in the field. Just like with normal companies. If you pay a small dev team for a couple of years to create a system that seriously speeds up or improves your experiments, that is worth a lot of money since it will allow you to get more high level papers, hence more grands. If you would just open source it, other labs (i.e., the competition) can use it as well and the grands might go to them.
And another reason: for experiments that matter, you need instant support and bugfixes, cause time is money. For example a couple of hours of using an MRI scanner easily costst thousands of dollars. So if something goes wrong, it needs instant fixing. You cannot rely on an open source ommunity and goodwill for that.
Of course there are other types of experiments/projects for which none of the above really matters; also there are projects like PsychoPy which seem to gain some traction and definitely are open to helping hands.
I would love to donate some of my free time helping a project in which I believe with software engineering expertise. Heck I might even pick up some domain specific knowledge.