That only makes sense if models were autonomously adding MCP servers and managing accounts themselves. Currently, users add MCP servers to their clients (like Cursor or Claude Desktop), so authentication should happen at that level. If the model handled auth, you'd still need to provide credentials to the LLM for it to provide it to the MCP server anyway. It's better to pass auth data as for example headers in requests - this way your credentials never get sent to the model provider (unless you're running locally).