Why can't the Schrödinger equation be derived?

Honestly, I don't get it. People say it's because it's a postulate. But, I mean, I see people deriving the Schrödinger equation with the help of the wave function, $T+U$ and partial differentials in three space coordinates and one time coordinate. How is that not a derivation? And why can't the Schrödinger equation be derived?