You don’t have to build it from scratch
One thing I've noticed about organizations that are happy with their open source tools: they stay abreast of the plugins and extensions that are available from the community, and they make effective use of them.
Drupal has over 40,000 available modules
WordPress, over 55,000 plugins
CiviCRM, over 500 extensions
Whatever your unique need, there’s a very good chance that you can get it done with creative use of some of these freely available tools.
Using tools from the community has distinct advantages over building your own:
It challenges you to think about your problem in a way that makes sense to lots of people not just you.
Most of these tools are going to continue to be improved by the community for a long time. That includes security updates.
When you need help making it do something new, there are lots of people who can help you.
Yes, sometimes your need is truly unique, and in that case building it from scratch with custom development is certainly an option — if you have measuable business goals that will justify the expense of creating and maintaining what you build.
But if there's a tool right there waiting to be used, why oh why would you reinvent the wheel?