That's part of their personality. Not that I don't have any criticisms of that episode. He comes into Homer's life and is absolutely a perfect person and saves Homer. No, he doesn't. He sort of leaves.
Homer steals money from the insurance fund in order to grow hair.
The character, Carl, pays the money back and says he was the one who committed this fraud and gets fired. Carl's a good person, but he's not altruistic. Even the group Jews for Jesus has appropriated Bart. Of course, anytime you create something and somebody else appropriates it to get across a message you don't agree with, it's irritating. On the other hand, the black Bart T-shirts are hilarious.