How is it that when a child goes to different profession psychologists there are way to many differences in th?

my child had been to many dr's when he was younger, and always a different diagnosis. He had been to psychiatrists and psychologists and still never the same diagnosis and still the problems remain. He has been in a mental ( rehabilitation center) hospital for 7 yrs and it's still going on, He has seen so many differnt dr, and still no correct or same diagnosis? Why is that?