That's libertarian dogma that elides the whole world outside of its narrow logic.
Corporations are ultimately meant to be agents that further the public good. They're not vehicles for the unrestrained pursuit of private profit to the exclusion of all other concerns. If natural economic incentives are not enough to motivate them to act in the interests of the public, then its the job of government and its regulations to compel a solution. To wit: if bank consolidation causes public goods such as community lending to be neglected, then government should forbid bank mergers and break up large banks until it no longer is.