Because it could very well exceed our capabilities beyond our wildest imaginations.
Because we evolved to get where we are, humans have all sorts of messy behaviours that aren't really compatible with a utopian society. Theft, violence, crime, greed - it's all completely unnecessary and yet most of us can't bring ourselves to solve these problems. And plenty are happy to live apathetically while billionaires become trillionaires...for what exactly? There's a whole industry of hyper-luxury goods now, because they make so much money even regular luxury is too cheap.
If we can produce AGI that exceeds the capabilities of our species, then my hope is that rather than the typical outcome of "they kill us all", that they will simply keep us in line. They will babysit us. They will force us all to get along, to ensure that we treat each other fairly.
As a parent teaches children to share by forcing them to break the cookie in half, perhaps AI will do the same for us.