The problem is that even if the code is clear and easy to understand AND it fixes a problem, it still might not be suitable as a pull request. Perhaps it changes the code in a way that would complicate other work in progress or planned and wouldn't just be a simple merge. Perhaps it creates a vulnerability somewhere else or additional cognitive load to understand the change. Perhaps it adds a feature the project maintainer specifically doesn't want to add. Perhaps it just simply takes up too much of their time to look at.

There are plenty of good reasons why somebody might not want your PR, independent of how good or useful to you your change is.