I don't think there is any guarantee that HTTP will always be involved. For example I might be calling a local LLM via CLI/script on a server with a stdio MCP connector that just runs other CLI commands, and never sends any HTTP traffic.

gRPC spans layers, and it uses HTTP in a more intrusive way than even MCP does — it expects to own the entire URL space at the IP/port in question. Using gRPC in a nontrivial way for MCP would be fairly heavy-weight: you would probably need to set up reflection and figure out how to bind all the MCP calls to it unless you just use it as a tunnel.