Because I test everything thoroughly at the REPL as I am developing, and record my REPL tests. Because I have been doing this for 35+ years and generally have

Because I test everything thoroughly at the REPL as I am developing, and record my REPL tests. Because I have been doing this for 35+ years and generally have acceptably low error rates without it. Because I would rather invest test resources in runners that actually exercise the application at the UI layers and the API layers directly in a fully configured and deployed environment rather than trust mocked up unit tests. (Which CI can also do.) Because tests can have as many bugs as the underlying code. But, for teams of junior developers, I am all for it. (I also will write some test harnesses/unit tests for aggressive or major refactorings.)