It's because software that solves the problem the business intends to solve, and can be maintained without unreasonable time and effort, is good enough. Code readability and maintainability problems are not business problems unless they impact developer productivity to the extent that feature development becomes too slow to tolerate.

After all, if work inside the codebase does not make a difference to the people who use the software, in terms of reliability or features, is it really worth doing?

The trick is to find a balance where efforts to improve code quality actually improve outcomes for the business and the users -- otherwise it's not justifiable to take time away from feature development, which is what the users actually care about.