1. The XNU kernel isn't novel, and isn't used by anyone other than Apple. Open-sourcing the NT kernel would effectively kill the project, since everyone would port the Win32 bindings to Linux (same thing would happen if Apple open-sourced their userland software)

2. Apple didn't get to choose whether they could make LLVM/Clang open source. They were forced to release it under GPL, and if you actually look at the license for those projects you'll see that Apple's contributions are dual-licensed.

3. What chance would FoundationDB have if it wasn't OSS? Why would Apple maintain a stale, internal fork of Apache Cassandra?