Because you need mainly a bridge between the Function calling schema defined that you expose to the AI model so you can leverage them. The model need a gateway as API can't be used directly.

MCP core power is the TOOLS and tools need to translate to function calls and that's mainly what MCP do under the hood. Your tool can be an API, but you need this translation layer function call ==> Tool and MCP sits in the middle

https://platform.openai.com/docs/guides/function-calling