Ego. It's really difficult to say "I was wrong". Hate to be so callous about evaluating an NBA player. Evaluating NBA talent, except in the rare cases of the Wemby's and Lebron's, is extremely difficult. Unfortunately, they get it wrong more often than right. Even guys like Sam Presti.
No disrespect to the players involved, but imagine if BOS had over-valued Romeo Langford. 14th overall pick - flashes of promise.... But you are right, every team gets it wrong. I know I'd be horrible at that job, I get too emotionally invested.
Oh man, I'd probably so bad at it cause I root for every one of them to succeed. I still watch Carson Edwards highlights!