I've done a handful of tests over the years. Once in a while the original didn't have any tests and I had no confidence I could change it without writing tests. Once in a while the original was buggy and after getting tired of going back top fix bugs I wrote a few tests. Once I had a case where the fix for bug A introduced by B, which the obvious fix was revert this code that made no sense thus bringing back bug A - when someone realized this was happening every year we write a few tests just to stop that pattern.

The above those is a very rare exception. The general rule is once code is shipped management doesn't allow you time to make it better.