Why do wireless keyboards only use 128-bit AES? Why not 256? I've looked for 256-bit AES encrypted wireless keyboards, and apparently they don't exist. Can anybody explain why this is from a cryptographic point of view?

Why wouldn't they just opt-in to the highest standard of encryption, instead of settling for the runner-up? Is there any cryptographic (or closely related) reason why industrial producers might have decided to use AES-128 instead of AES-256?
AES-256 isn't "better" than AES-128 in any practical sense. A crack of AES-128 will almost certainly bring down AES-256 anyway, and there's no practical difference between 128-bit keys and 256-bit keys — nobody will crack a 128-bit key in the future of humanity as a species.

On the other hand, AES-256 likely requires more power draw than AES-128 for this piece of hardware. I'll take an extra hour of battery life over a meaningless difference in security. Hell, I'd take an extra five minutes of battery life.