Architecting the core of your game from the beginning is an often-skipped process. Many developers are too eager to build their game and just start placing assets in a scene as they go. This kind of practice is fine for prototypes (mostly, however, even with prototypes, a level of architecture is usually required). When building your actual project, however, without setting up a proper architecture from the beginning, you are heading toward a world of utter mess.
When we say architecture, it doesn't mean that you need to design everything (but it helps). You just need to ensure that you plan what you are going to build before you build it instead of thinking about stuff and checking Google for information on how to do it. Even if you are using some kind of an agile method, you should have a good framework and goal for each step. This will guide you on what should be done and when, not just designing the project on the fly.