In theory you can write your own "everything" but it takes to much time. For learning point it is great aproach to create mocks of used and loved tools there are many tutorials in form of "create your own x". Making you both understand the tool and improve your development skills. But in engineering stand point if there are a good tool that is suitable for your need and available for your use it is better to use it. And spend your energy to learn how to use it efficiently. Since they have many time tested features and learning about them improve you too. You can think as people using test tools and frameworks to develop big projects. Incase of gamin studios who develop their own engine they either need specialized system or due to legal/cost/ownership reasons. So if you dont have such problem general approach is to use trustable tools.