Because the source code is free and open to everyone so it literally means that you won't steal it. And you want to create an original bitcoin? we have already one and that's what you are trying to fork so don't call your idea as an original one if you're just copying the code and you don't have anything to add valuable with it. And you know what? we've got plenty of fork coins already, you can count them alone by just looking at this site: https://forkdrop.io/how-many-bitcoin-forks-are-there