Rehabilitation of cheaters.

And yes, in cutting-edge technologies, the security system is essentially ALWAYS open. Only the key should be closed! Hiding the encryption system is an indicator of its vulnerability.

But that's not the case; read about fully homomorphic encryption and zero-knowledge proofs.

Plus, the vast majority of computer systems are proprietary and even obfuscated (serious systems - SCADA, ATMs, OS kernels, for example, not just computer games).