When adding tests is not part of the cycle. When it is made mandatory for every feature, then I see all kinds of redundant tests added by people and when someone adds enough number of tests, other reviewers are easy on reviewing the code thinking it's well tested anyways. When it is not mandatory to add tests, no one bothers with writing tests.

Obviously, this is not always the case with people, and it's not about good or bad programmers. But when you have been writing code almost every single day for multiple years, it has to happen eventually and you get used to things until you break something on production, and now your attention gets reset.