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The Event Loop doesn't fetch from the Callback Queue when there are no items left in the Call Stack because the Call Stack is the execution context of the currently running function. The Event Loop is designed to check the Call Stack first for any pending functions to execute. It only checks the Callback Queue when the Call Stack is empty. This ensures that the execution of the current function is completed before moving on to the next one. Therefore, if the Event Loop were to fetch from the Callback Queue when there are no items left in the Call Stack, it could potentially interrupt the current execution context, leading to unexpected behavior or errors.