isnt that what they are hired to do though?
No. All they can do is gather as much info as they can and make the best choice possible. There is no way they can know how a player who has never played in the NBA is going to react once he's there.
They can get an idea. But at the end of the day, they get their info, learn as much about the guy as possible and hope they did the right thing.
There are also a lot of things that they simply can't know that can also effect how a player performs.