WHY Sex is physical. Sex is the shape and function and form of physical organs of the body. Gender is a subset of basic identity, a mental and emotional moiety, without an easily defined physical existence. Although our identity, and therefore our gender, reside in the neurochemistry of our physical brains, it is impossible to point to the precise location of either or to quantify them in a purely physical way. Identity, and gender, are the abstract result of the complex interactions of a vast system, not a specific thing.
Gender is part of identity of self.
An individual person possesses an individual identity, and that identity is defined by a set of describable qualities, one of which is gender. If a person were to have their gender changed, they would be also changing their core identity. A different identity, by definition, would mean a different person. A change of gender would essentially mean the death, or annihilation, of one identity, one personality, and the replacement of that personality with a new one. Unlike 'transsexual' which clearly defines a condition, 'transgender' is a null term.