Why are MCPs needed at all?

I think that the answer is in the question. MCP's are needed to manage the MxN connector problem.

Whether "they" could have (re-)used some other protocol or protocol family (like OpenAPI) is unclear / debatable. But presumably, "they" didn't think so.

The IT industry has been inventing / reinventing application protocols since at least the 1970's. Don't expect this to stop ... just 'cos you think there is a better way :-)

GraphQL comes to mind as a resource descriptor language as well.

MCP is glorified switch statement

I agree with the sentiment that reinventing the wheel isn't something that we are experiencing just now, no argument there.