Ask HN: Piecing code together in your startup/projects

hello everyone. I was recently considering the fact that a lot of code that is needed to make a lot of applications have basic functionality, revolves around a lot of common things. Let us consider user signup and social signup. If you are using an MVC framework like Rails or Django, this has already been done for you. Consider another few features and you soon see that most of them have been well written before. Based on this, would it be viable for a startup to simply piece together these pieces of code in a sort of "hack" to get a MVP out there? I can't seem to understand any reason why a startup would want to write so much code in the beginning when these type/s of resources exist. Any thoughts?