I'm not sure about this - there are many open processor designs in academia if a fb/google wanted to pick them up - the difficulty is integration and software. They could easier just work on ARM, the reference designs are available if you are fb or google.

I guess what I'm saying is, even if a reletively modern, 2-issue, OoO, with SMT and 256b vector proc, came out open source, would anybody really bother to integrate it and fab it?

From what I see fb and Google work with silicon vendors because they don't want be silicon vendors.