It's a protocol that doesn't dictate how you are calling the tool. You can use in-memory transport without needing to spin up a server. Your tool can just be a function, but with the flexibility of serving to other clients.

Your not spinning up a whole server lol, most MCP's can be run locally, and talked to over stdio, like their just apps that the LLM can call, what they talk to or do is up to the MCP writer, its easier to have a MCP that communicates what it can do and handles the back and forth, than writing a non-standard middleware to handle say calls to an API or handle using applescript, or vmware or something else...