A defense attorney is supposed to defend both innocent and guilty clients. The writers of Ace Attorney know this, because there is an attorney in the game who specifically defends guilty clients (and is portrayed positively).

Phoenix Wright’s moral superiority complex and black and white thinking, embodied in his refusal to defend guilty clients is a deliberate, core personality trait of the character.

Also simply in terms of methodology, he almost never actually defends his client but rather prosecutes an unindicted individual whom he had not yet identified.

Are we shocked that the art student who became a lawyer for ulterior motives sucks at his job?