There are more constraints than that - the corresponding words in each phrase are also the same parts of speech, and are often read with a specific cadence. See

There are more constraints than that - the corresponding words in each phrase are also the same parts of speech, and are often read with a specific cadence. See https://en.wikipedia.org/wiki/Antithetical_couplet https://en.wikipedia.org/wiki/Antithetical_couplet