Why write your own CMS?

I've been surprised to still recently find software development agencies using their own proprietary content management systems and frameworks.

With so many available open source options, like Drupal and Symfony, why would people write their own?

Why spend the time and effort to build basic functionality like user registration and login when existing tools already solve that problem?

Why ignore the thousands of open source modules, themes, plugins, packages and libraries that are available?

Instead, leverage the tools that already exist and focus on solving domain-specific issues and writing code that's genuinely specific to the project.

Then you can contribute some of the saved time back to the project and make it better for yourself and others in the future and ready for the next project.