Agreed, but there have been big projects that have been open source. I can imagine* an AI (LLM) being developed fully FOSS. It would be rare, but I can see it happening if a big foundation got behind it. Maybe Mozilla, or another that tries to keep the spirit of their mission statement.
*Imagine: I'm not too familar with all of the current, public, and free models out there, just a few. This was just me making a hopeful guess about if it might be actually happening now.