Even if they have the source, they may not have all the build tools anymore.

Or they have the build tools but the wizard that set up the build system back in the day no longer works there.

Or they have the build system archived and documented but it doesn't run because some license expired, and the tool vender doesn't sell that version anymore.

In the near future, there will be another possibility - SaaS cloud tools that are impossible to preserve so they are forever lost.