Because again, discoverability is baked into the protocol. OpenAI specs are great, but they are: optional, change over time, and have a very different target use case.

MCP discoverability is designed to be ingested by an LLM, rather than used to implement an API client like OAI specs. MCP tools describe themselves to the LLM in terms of what they can do, rather than what their API contract is.

It also removes the responsibility of having to inject the correct version of the spec into the prompt from the user, and moves it into the protocol.