Why is this not a more common way for generating 12 words?
Your seed phrase represents a large random number and the standard method is that you first generate that random number.
There's nothing preventing you from going to the word list first directly. If you use a method in which words are selected completely randomly, you can achieve the same security.
I intend to flip a coin 121 times, convert to BIP39 words and then enter them into my HW with a random 12th word until it accepts the mnemonic as valid (passing checksum). This way I'm not relying on the HW RNG.
With flipping the coin 121 times, you actually generate a random entropy. It means that you use the common method and the only difference is that you generate the last 7 bits in a different way.
By common method, I mean generating the entropy and then go to the word list.