Bear in mind, in discussions with the AMP tech lead, I've discovered AMP4Email isn't even being approached as a standard. Gmail is implementing it as a proprietary fork of email, and the AMP Project is just deciding whether or not they'll support it directly.
It seems to me that Google only submits to the W3C when it needs other browsers to implement something. Whenever Google's monopolies can handle it, they keep it proprietary.
Perhaps there's a perception that the W3C is premature at this stage, as things are quickly evolving and standards committees usually standardize stuff after there's been some proprietary experience.
The W3C might not even be the right venue. For example, changes to JS go through TC'39, whereas changes to protocols go through IETF. Lots of XML/SGTML spec work has been done outside the W3C. I don't feel knowledgeable enough to make an informed comment.
I'm not a fan of some of the aspects of AMP, but I feel like somebody needs to try something new and stir the pot a bit.