People forget, Walt was very unhappy and depressed with his life and felt like a failure prior to cooking and selling meth. Now he's in a position where his hard work and talent is heavily respected and not to mention he's been wildly successful. Going from this, back to his mundane, boring life would be torture for him. That's motivation enough for him to stay in the business. It's not about the money or his family - it never was - it's about getting the respect and success he felt that he has always deserved. He'd rather be a king in destitute than a wealthy peon.