A couple reasons come to mind. (1) you can't separate the software from the rest of the device: it has to be Approved as a whole. (2) certification isn't enough

A couple reasons come to mind. (1) you can't separate the software from the rest of the device: it has to be Approved as a whole. (2) certification isn't enough. e.g., you could submit the software to a quality agency who would certify that it meets IEC62304, but then the entire device and its clinical test data still has to be submitted to the FDA, along with its Quality Record and all that (submission process, maintaining the Design History File, etc.) costs Real Money. And if you manage to get it approved for sale, then there are regulatory requirements around support and maintenance of the device that aren't going to happen for free. It's a nice idea, but unless you get funding from a philanthropic billionaire, I don't see it happening