Unfortunately, all of the above methods are beatable (sometimes trivially), slightly intrusive on the end user, and if they malfunction they can even lock a legitimate player out of their own game. Instruction manuals can be photocopied (despite efforts to make this difficult) or just plain lost, physical game discs age and eventually go bad (making perfectly-legal archival/personal backup copies won't help if the game uses a key-disc method), and so on. Sometimes, a method becomes viewed as so intrusive that the player may simply choose to avoid running the game at all... or decide "screw it" and download a cracked, pirate version, thus leading to the exact opposite of what the publisher intended.
This has resulted in something of a vicious cycle between game publishers and unlicensed copying ("piracy"), where when one copy-protection system is cracked or beaten, the publisher must switch to (or create) another, stronger method, which is itself beaten (sometimes quickly), and so on. Where does it end? 2b1af7f3a8