For any individual RHEL package, you can find the source code with barely any effort. If you have a list of the exact versions of every package used in RHEL, you could compose it without that much effort by finding those packages in Stream. It's just not served up to you on a silver platter unless you're a paying customer. You have M package versions for N packages - all open source - and you have to figure out the correct construction for yourself.
There's nothing special or proprietary about the RHEL code. Access to the code isn't an issue, it's reconstructing an exact replica of RHEL from all of the different package versions that are available to you, which is a huge temporal superset of what is specifically in RHEL.