Because that is what the law and those licenses on the box say (the EULA if you remember the times when physical software came with a "read before opening" license agreements).
There was no licensing with any physical media I bought. It had the FBI thing and a piracy thing but my country isnt obligated to enforce that. They were legitimately sold in my country with no mention of a license.
Only recently, like ps4 times have I had to tck a box when playing a game.
GOG tends to be my prefered source of games. DRM free and i have the option of making my own backups. I never felt the need to though. And i love the convenience of being able to download my games.