Anyone can halve at any time, so anyone can "cheat" that by halving immediately.
The reason everyone does it at the same time is because doing it [literally right now] will lead to a history that nobody else agrees is valid (halved at the wrong block number), so any coins you mine on your forked chain will be worthless. There's a monetary incentive to play by the rules, but absolutely no technical requirement.
Bitcoin's primary achievement (IMO but it's fairly common) is that it managed to design a technological system that encourages playing by the rules. Cheating always pays worse than playing along, and even trolling only works if you have the majority of all computing power (very expensive), so there's no reason to cheat. But outside the core public key cryptography that handles addresses and proving transactions, and the "proof of work" that basically just limits the speed of everything so there's time for the world to agree on things, there's not really any fundamental crypto involved. Just self-reinforcing social incentives.
(this kind of disagreement is why there's both Bitcoin and Bitcoin Cash. they share a common beginning but branched off some time ago and are now completely separate)
The reason everyone does it at the same time is because doing it [literally right now] will lead to a history that nobody else agrees is valid (halved at the wrong block number), so any coins you mine on your forked chain will be worthless. There's a monetary incentive to play by the rules, but absolutely no technical requirement.
Bitcoin's primary achievement (IMO but it's fairly common) is that it managed to design a technological system that encourages playing by the rules. Cheating always pays worse than playing along, and even trolling only works if you have the majority of all computing power (very expensive), so there's no reason to cheat. But outside the core public key cryptography that handles addresses and proving transactions, and the "proof of work" that basically just limits the speed of everything so there's time for the world to agree on things, there's not really any fundamental crypto involved. Just self-reinforcing social incentives.
(this kind of disagreement is why there's both Bitcoin and Bitcoin Cash. they share a common beginning but branched off some time ago and are now completely separate)