From the perspective of a software author, I am not at all surprised.
If you are a user reporting a bug, and you say "I am using the RPM package of systemd from CentOS 7.4", then I know which RPM to download to get exactly the software you are using.
But, if you say "I am using the RPM package of systemd from CentOS 7.4, plus my own patch", then things will be harder for me. Not only would I need to get your patch, to be completely safe, I would need to get your build environment. For example, which compiler you are using, and which version of the compiler.
Working off of a common set of pre-built packages makes problem reproduction alot easier.
If you are a user reporting a bug, and you say "I am using the RPM package of systemd from CentOS 7.4", then I know which RPM to download to get exactly the software you are using.
But, if you say "I am using the RPM package of systemd from CentOS 7.4, plus my own patch", then things will be harder for me. Not only would I need to get your patch, to be completely safe, I would need to get your build environment. For example, which compiler you are using, and which version of the compiler.
Working off of a common set of pre-built packages makes problem reproduction alot easier.