People end up doing this because code gets written incrementally and often we start out wrong (e.g. here with the wrong/inverted condition). Rewriting large code blocks for a little more clarity is often a PITA.

It would help very much if editors supported this better (e.g. single keystroke inversion of an "if"). Go with its easy syntax would be a particularly good target for automatic rewrites for code like the above, or even just editor-level hints for better style.