This is a real problem in our industry. We all like to think that we 'know' how to write good code, after all we read 'Clean Code' once. That, combined with either overconfidence in our abilities or the negative perception that comes with not knowing the answer to every problem leads to a lot of software that has applied 'good' coding practices and architecture to produce a mess of awful code.
In reality it is probably a lot more like learning how to apply any other concept, it takes practice, yet we don't spend enough time practicing our craft. This is particularly a problem because in the course of developing a product you will only get to implement a new solution to a problem a few times at best, or once more likely. This means we don't get enough experience with the different possible approaches to really internalize what a 'good' solution looks like.
We spend much more time learning the software development approach du jour, XP, Scrum, Kanban, Lean, etc. It doesn't matter what software development approach you use if the output is an unmaintainable mess of code.