But everybody kinda knows that CC0 is more or less in the spirit of the OSD (so long as there are no patents involved) and OSI doesn't go around hollering about it, because it's not that important.
As opposed to the Commons Clause, and other historical attempts to undermine "open source" as being nothing more than "source available" — those are battles worth fighting. The OSD is more important than OSI. If OSI were ever to change the OSD in a substantive way, there would be a huge revolt against them.
From https://opensource.org/faq#cc-zero
> CC0 was not explicitly rejected, but the License Review Committee was unable to reach consensus that it should be approved, and Creative Commons eventually withdrew the application. The most serious of the concerns raised had to do with the effects of clause 4(a), which reads: "No ... patent rights held by Affirmer are waived, abandoned, surrendered, licensed or otherwise affected by this document.". While many open source licenses simply do not mention patents, it is exceedingly rare for open source licenses to explicitly disclaim any conveyance of patent rights, and the Committee felt that approving such a license would set a dangerous precedent, and possibly even weaken patent infringement defenses available to users of software released under CC0.
So it's more that there are concerns and nobody has cared enough to fully resolve them to arrive at a clear answer. It's also not like the CC licenses are commonly used for software so this is not surprising.
For example: https://opensource.org/node/878
> it’s impossible to make a globally applicable statement that a certain piece of software is in the public domain
Not that something truly in the public domain is not open source.
