Yes. Random key it is the best decision, but problem that it is easy forget this random key.
Forget? Normally human can't even remember number with length above 10, let alone private with 256-bit length (or 64 if you try to remember HEX format)
You should keep this random key or feed in other place than your mind, so it is additional risk.
To be fair, any option have it's own pros and cons.
But it's most common practice (except we randomly generate xprv/seed) & you probably use this method as well.
Sure. It is absolutely right.
But we are started talking about brain wallets and brain wallet feed could be generated randomly. I can keep in mind 16 random generated words, but problem that words are already existed and could be generated again. Good way to change 1 word from this 16 to your own created word.