It's not all that difficult for a good piano player to play lots of notes rapidly all over the keyboard. But a tuba player can't play complicated melodies anywhere near as rapidly. And some things are impossible to play even for an excellent piano player. Trombones and timpanis are famous for this capability, but many other instruments also have amazing capabilities. Judicious use of pitch bending can sometimes be used to make an electronically synthesized midi file sound realistic.
String instruments are by far the most difficult to make realistic.
A violinist can play many different ways, with different attacks, different degrees of vibrato, and so on. A string section in an orchestra contains many instruments, each played slightly differently. During lush orchestral passages, the net effect is to smooth out the edges. The key point here is to make the quality of the strings change continuously. Of course, in midi, one can synthesize new electronic instruments that simply aren't played by live musicians.