Because they're keys to different algorithms. The bit-length in AES is not an arbitrarily tunable parameter. AES-256, AES-192, and AES-128 are related but non

Because they're keys to different algorithms. The bit-length in AES is not an arbitrarily tunable parameter. AES-256, AES-192, and AES-128 are related but nonetheless different algorithms. The recent attacks on AES-256 and AES-192 have exploited the key schedules, which is one component that's different between the three algorithms. AES-128's key schedule is better-designed.