A mnemonic sentence constructed from random inputs is much more unique than most people can possibly imagine. For example, in a 24-word BIP39 sentence, there are 2048 to the 24th power possible combinations of words (or stated another way, around 3 x 10 to the 79th power). To try to comprehend how enormous this number is, it's often compared to an estimate of the total number of atoms in the known universe. In practice, it is effectively unguessable with today's technology.