Your analogy is a gross distortion and ignores what you have been told. When you are answered in a single sentence, you need to pay attention to all the words in the sentence. I have bolded the word you ignored. Quantum Field Theory and General Relativity are fundamental physical theories because they explain the most general behavior of physical phenomena without reference to any more generally applicable physical laws. As only General Relativity speaks to gravity, then there is no more fundamental physical theory about gravity than General Relativity. Before General Relativity, the former theory of gravity was Newton's Universal Gravitation and when asked for a mechanism Newton wrote: because fundamental physical theories describe only behavior not mechanisms. Why? Because behavior is the only phenomena we have evidence for if the theory is fundamental -- any number of mechanisms may be proposed to "explain" a behavior and evidence doesn't let you choose between them when no evidence of any more fundamental mechanism is known to mankind.