Sure, but say you're Intel and pitching the technology to Hollywood. Open source would make the entertainment industry nervous. And Intel isn't in a bargaining position, since Hollywood would be more than happy to just shut x86 PCs out of content.
(Needless to say, I'm offering an explanation as to why the Intel ME is what it is, not defending it. I think it's pointless. After all, it's likely that PCs are just not going to get content in the future, given that consumers use set-top boxes that use embedded architectures instead of x86 PCs and Intel has failed in mobile.)
I'm pretty sure only its keys really need to be secret, but hiding the code may provide some extra security by obscurity if the code happens to have bugs.
Does it, does it really?
I'm pretty sure security through obscurity is some bullshit.