Amazing. I had exactly the same reaction a couple of years ago when I attended the Busoni competition in Italy. The finalists were required to play a Mozart concerto, and they were almost uniformly dreadful: loud, bombastic, no phrasing, ugly ornaments. These were the same pianists who had given spectacular performances of Prokofiev, Rachmaninov, Liszt et al. The Mozart was obviously ignored in the judges’ ratings.
On the other hand, I have heard children perform Mozart superbly at various music schools in the Boston area where I live. They played with spontaneity and ease, without the technical hangups that we all seem to develop as we overtrain and overpractice. And they seemed to enjoy it.
I think Mozart is difficult because:
You have to love every note, because technique and experience will not carry you through.
At the same time, your basic technique has to be flawless, even to execute the simple scales and arpeggios, which are so exposed.
Even though your playing must be flawless on the surface, you also have to articulate the underlying drama (like an opera).
There are too many things you have to do perfectly, with no safety nets, and yet you have to be spontaneous.
I think the piano parts of the violin sonatas are among the most difficult pieces to play.