It's much easier to build something that's suitable for your one specific use case than it is to build a general-purpose framework. Database systems, cars, and Bootstrap are all general-purpose. But if you need to store a collection of name/value pairs, carry a load from fixed-point-A to fixed-point-B, or create a web-page with a specific and custom style, building something from scratch can be the right way to go, and it can be easier than forcing a general-purpose solution to fit what you need.