Even the protection code in the 90’s had self-modifying code. It’s not novel or specifically hard to work with when you’re used to it, especially with modern tooling.

What makes some games harder to crack is that testing that they work okay throughout. That might mean playing all the game from the beginning to the end, and trying all the scenarios, and fixing all the issues found. Assuming that RDR2 takes at least 20 hours to finish, and close to 100 hours to fully complete, that’s a huge undertaking of course. It’s no surprise that it took that long.

The real reason is that executable modifying its own code is often flagged by AV, or the OS itself, as an "insecure" activity. Since self-modification is used in attacks and exploits, good protections rarely use it now. It's impossible to use codesigns with self-modified code.

Look into Blizzless :-)