This is the main reason for the simplicity of key checks.
If you copied someone elses windows installation (cd burners were very expensive, but a floppy version existed), you'd also copy the serial number, and just use that. Even if the serial verification algorithm was complicated, noone would really have to crack it at all, because they'd just use the original serial for all the copies.
Piracy was (and still is) rampant with home users, but business users still needed some kind of a simple check (so.. a simple serial) to match the installation to an actual licence, which was more "protected" than the actual medium (cd, floppy), by using holograms, microprint, etc: https://media.karousell.com/media/photos/products/2018/09/13...