Because the way you define success is, to solve a problem with a great result, a minimum of fuss and the least expense. If that is your definition of success, it has to be ours too. It also enhances the image of the legal profession. That is not true. Pit bulls are vicious dogs that are out of control.
Juries sense it too, and in front of a jury nice guys truly do finish first.
There is a place, however, for a display of righteous outrage and even a little yelling and screaming. An aggressive negotiator will walk all over you at a bargaining table if you don't meet him or her with equal intensity. The best lawyer is one who is genuinely a nice person, but who can be firm and tough when necessary, and always be in control of his or her emotions. It is like going to the dentist. You go to a dentist when you have a sick tooth.