Testosteron, growth hormon seems to have less influence on protein synthesis in skeletal muscle cell opposed to amino acids, but why it this?

T and GH seems to have not so much influence on MPS as sometimes is claimed. Looking at mTOR and MPS it seems many variables are influencing this pathway. So understandable a bit of T ang GH or insulin, in normal athletes will have lesser effect on MPS. However, the amino acids (or certain of them) seem to have a higher influence on MPS. Is that purely because they are building blocks for muscle or is there more to it?