As a developer, I frequently tell higher ups that "I have a conflict of interest" when it comes to testing. Even though I fully attempt to make perfect software, often I have blind spots or assumptions that an independent tester finds.

That being said: Depending on what you're making and what platform(s) you target, developer-owned testing either is feasible or not. For example, if you're making a cross-platform product, it's not feasible for a developer to regression test on Windows 10, 11, MacOS, 10 distros of Linux. In contrast, if you're targeting a web API, it's feasible for a developer to write tests at the HTTP layer against a real database.