Also Claude Code is very good at writing small scripts/on-off test cases to confirm bugs, so I wouldn't even say the initial premise is correct.
Sometimes you can do that mentally and fix the code. Often your fix will be right especially in a relatively simple part of the code. However, equally often you'll fix a different problem (or something that wasn't a problem at all), and the original bug will remain but you'll believe you corrected the issue. This is why you should always replicate a bug to understand it, and why you should always add a test whenever you fix a bug to prove you actually fixed it as well as preventing future regressions.