What about Ledger ones that have 24 words?
They start by using a (pseudo) random number generator to create a very large random number... this large random number is then converted into words by following the BIP39 process. (Note that Electrum follows a slightly different method for converting to words, but the underlying theory is the same... large random number -> words).
This is completely opposite to a "brainwallet" where the words are chosen first... and the number is generated from the words. As was already stated, creating your own brainwallet words generally leads to very poor entropy because humans are terrible at creating "random" things.
So, by starting with a random number, then converting to words... you retain the "entropy" to safeguard your coins.