My understanding is that the specification changes over time, and if your implementation doesn't match the central one, then the rest of the network stops interacting with you. That is: there is a massive network effect of people using github.com/bitcoin/bitcoin as canon, and there is a central group of devs with write-access there. While you can fork it, unless the rest of the network gets in on it, you've just created some altcoin that nobody cares about.