Game copy protection was an art form, as well as #cracking these protections. This cat and mouse game evolved over time, with protections including more and more complicated and well-hidden checks trying to identify poor or quickly made cracks. The best protections hindered the release of a crack long enough for the game to hit enough initial sales. After the initial sales the eventual release of a crack would no longer hurt (much).
Rewind to 2025 and legal purchase of early 90s PC games from places such as COG: Apparently, there are plenty of releases that have the poor crack built-in, and where the anti-crack checks kick in, making the game unplayable. The irony is that the only way to play is to get a proper "real crack".
Cool write-up of the protection in "The Games: Winter Challenge" and the associated problems of "legal releases":
https://mrwint.github.io/winter/writeup/writeup.html