As a developer, your job is to develop the software. Yes, you should be testing the code you write as much as reasonably possible. But since you wrote the code

As a developer, your job is to develop the software. Yes, you should be testing the code you write as much as reasonably possible. But since you wrote the code and have been working with the code very intimately, it's so easy to develop blind spots that might be obvious to a someone else but you just don't see because you hadn't considered it. That is why it's so valuable to have dedicated QA testers.