You download the upstream src rpms, build them, but by default they link to different versions of libraries than upstream. While this doesn't necessarily mean it will be buggy or behave differently than upstream, it might. Now you have to go in and fix that. And you spend hours doing it for a build. Then rebuild and repeat.
As I understand it, if they release with it wrong, it can be next to impossible for them to release a fix and have yum upgrade to the fix. They have to have package numbering exactly like upstream due to third party packages that rely on those numbers, so they can't bump a number to get yum to upgrade a package. So, they want to make sure it is perfect before releasing it.
I have no inside knowledge of the project, so I won't speak beyond that.