I think many devs build applications only testing manually for a long time, and taking shortcuts (hacks) when something appears wrong. When they later want to write some unit tests, there are no proper units and that ball-of-mud-y code is hard to test. Of course, integration tests are still feasible because they're agnostic of the internal mess.

I've run into this a couple times and noticed at some point that writing unit tests while developing (not necessarily TDD) helps a lot in clarifying boundaries early on, and generally improves code quality.