Could they be expanded/adapted, published for free, and put downward pressure on the (ludicrous) price of textbooks in the US?
1) There's not much legitimate value in the textbooks we have now. Those who did that work don't really deserve to make money from it, and their labor would be easy to replace.
2) It'd be free or essentially free to "expand and adapt" military documents and make them into textbooks that are accurate, current, and suitable for educating non-military folks.
2) You're right, no such project has ever existed; I'd imagine it would be like an encyclopedia, full of people who donate their time to edit page about various subjects. Such a project could never stay current, definitely isn't suitable for education, and almost certainly will never surpass the accuracy or magnitude of my Microsoft Encarta CDs. [0]