Giovanni's role as the Boss is a front for him. It isn't his life... in fact, it is far from that. Beign the Boss means little to him. The job is kept because it's his, and it's something to hold onto. When Maria offered the job, he accepted... eventually. She tested him daily, trying his patience, trying his wits. Finally, however, she relented that it was his job. That is, it would be his job if he could kill her.
He didn't want to kill her initially, and pleaded her to think sanely. She laughed at him, and he began to rememebr everything she'd done, everything she'd put him to. With that in mind, he'd shot her.
From then on, the job was his. At first it seemed to be exactly what he wanted. Everythign was going fine, everything made sense to him.
Then, suddenly, he found that it was nothing. Everything he had worked for meant nothing to the world. He thought it'd go away, but it didn't. Day after day it plauged at him. His past, his future, his position... It all bit into him sharply, unkindly.
Giovanni's job as the Boss is a display of the hardness of the world that beat into him throughout his life. It reflects his views of the world. Above all, it lets him vent.
It's all a mask, though. Just as the shadows veil his face from many of the grunts, the job veils his conflicting emotions.
As the Boss, Giovanni appears to be heartless and cold, a machine. He knows this, and has let it continue. He sees no reason to speak otherwise. The Team runs onward, and the agents listen. He knows what'll happen if they discover a way to gain control, and doesn't want to push it.
So he stays where he is. There's no where else for him to go. No one to him to go to.
He's alone, and he hates it, but being the Boss covers that... right?