Their design is pretty much solely a limitation caused by the idea that all filesystems should exist through the 9p protocol, which as a networked protocol cannot share data (fds, structs), only copy (payloads to/from read, write). With the idea that all functionality had to be replaceable and mountable from remote machines, the only possible API became read/write for everything.

It's not clear to me that 9p itself could not be extended to allow for shared memory. With low-level control over the operating system and rebuilding of existing binaries, distributed shared memory becomes a possibility. (I.e. the existing VM system ought to be enough to implement whatever cache coherence is needed for shared memory over the network.)