> The data/time overhead would be negligible.

Doubling or tripling the amount of data sent would be negligible over a wire, but an audio protocol won't be as snappy. Then there's the matter of trust/decryption. How are those keys being kept safe? What happens if I lose access?

Then don't double or triple the amount of data sent. There's simply no need.

You can encrypt data and keep the exact same original byte size.

If you're using symmetric encryption, sure. And then every dishwasher has the same symmetric key, guaranteeing it'll leak.

Everything uses symmetric encryption, including asymmetric encryption. Using symmetric encryption doesn't mean you'd use the same key for every dishwasher, you'd obviously just pair the devices and generate a new key, like everything does nowadays. Also, my dishwasher doesn't use any sort of encryption, and it still leaks.