You can do a lot of good in the world.
In fact, it's difficult to justify why you don't spend most of your time doing good. It's cheap to save lives. GiveWell puts the number at ~$3500. You could forgo many expensive purchases and instead save lives. Better than merely giving money, change your life plan and directly work on whatever you think is best. Depending on who you are, that may be better. However, you do it, if you became obsessed about improving the world you could. And the costs to yourself would be minuscule in comparison with the benefits to the world.
But most of us don't invest that much in doing good. We don't seek sainthood.
Susan Wolf argues that morality and meaning aren't the same. In fact, pursuing sainthood would risk making a life worse, not better. Being moral is only one good in a life, there's more to living a good life. By "meaning" here we're talking about how good a life is, the value of a life. There's fine wine, friendship, and the aesthetic. By living as a saint, you're sacrificing other virtues and goods:
If the moral saint is devoting all his time to feeding the hungry or healing the sick or raising money for Oxfam, then necessarily he is not reading Victorian novels, playing the oboe, or improving his backhand. Although no one of the interests or tastes in the category containing these latter activities could be claimed to be a necessary element in a life well lived, a life in which none of these possible aspects of character are developed may seem to be a life strangely barren.
Yet people who we consider supremely moral whether Ghandi, Jesus, or MLK seem to live lives full of meaning. Perhaps they aren't moral saints. all three were human. Nonetheless, the candidates have in common personal sacrifice and a narrow focus on living well.
Wolf's case extends not just to saints, but to obsessives of all varieties. Lyndon B Johnson with power, Steve Jobs' with iPhones, and Derek Parfit with moral philosophy. Such people live lopsided lives, sacrificing the value of friendships, being a present parent, and hobbies to their aspirations.
Wolf herself pushes back against this characterization. This is because the pursuit of being as good as possible is a higher-order desire, one that subsumes ordinary desires:
The normal person's direct and specific desires for objects, activities, and events that conflict with the attainment of moral perfection are not simply sacrificed but removed, suppressed, or subsumed. The way in which morality, unlike other possible goals, is apt to dominate is particularly disturbing, for it seems to require either the lack or the denial of the existence of an identifiable, personal self.
What is disturbing about the moral saint is that all of desire: whether for food, relationships, physical exercise are made to service the moral saints goal of doing good. This feature isn't unique to moral desires however. My sense of most obsessives is that they're quite familiar with modulating less important desires. Exercise becomes good because it helps one make better decisions. Relationships are good insofar as they're useful for other larger, goals.
So, I think it's best to understand Wolf's view as not just against sainthood but against obsession. Whether the obsession concerns morality or not doesn't initially matter.
Of course, there are people who become obsessed in dead ends and fail to succeed. Perhaps such lives are less meaningful. Moreover, corporate and political pursuits are not meaningful unless the end is ultimately valuable. The satiated power hungry politician may have succeeded in holding an office. But life isn't about offices. Yet, so long as you're pursuing a worthy goal, the life of the obsessed seems more meaningful than the typical person. They're revered in a way others were not. Even if it's true that many would not want to live like them.
The idea that there's more to life than obsession is a hedge. You're hedging against the risk that you're not pursuing anything valuable. You don't want to be the successful and lonely tycoon asking what was this all for. If you're obsessed you're moving all of your chips into a particular value. If you're wrong, you'll go bust and your life risks not being good at all. You're also hedging against the risk of failure. When I think of people who are saints it's obvious that they live moral and meaningful lives. Vastly more meaningful than the typical person. But the risk of failure is scary. Obsession does come at the cost of other values. When we think of saints, the one's who failed don't come to mind. But even here, I'm sympathetic to the Stoic view: if they were virtuous and acted as best as they could given what then knew, then that seems exceptionally meaningful.
Most will be happier to hedge than seriously risk the value of their life. But when they get it right, we're grateful to those who risked a lot.