It's important for managers of software development to pay attention to the quality of their output in addition to the quantity for the simple reason that poor software will always lead to future problems. Whether it is through organization changes, or attrition, it is likely that the developers that write a system may not be the ones who modify that system down the line. If proper focus is not paid to the quality of the code being written, the following types of problems can occur:
Poor code will be difficult for future developers to work with which will add friction to their work or onboarding and slow down future growth opportunities for the organization.
Poor code will scale poorly. A shortcut here, a shortcut there, and you can easily end up with a mess down the line that will cost support hours (both dev and infrastructure) and analysis and development hours to fix.
Poor code will make good coders write poor code. Much like the broken window fallacy, developers may not see value in putting effort into fixing garbage code, especially if the organization already does not give code quality importance.
There are lots of issues with not giving good code quality the importance it deserves. Great software managers should pay attention to it.