The problem of synthesizing a believable trumpet sound is way harder than creating a believable tube distortion. The trumpet is a complex system consisting of the the interplay between the horn and the lips, tongue, throat and lungs of player. Also, the way you interact with the trumpet is widely different from the way you play a piano, resulting in different musical expression.

Compared to that, the dynamic of a tube are pretty simple, though creating a truthful emulation of a full amplifier is still a hard.