That's the whole point of this thread - not using third party code to generate your entropy. You can flip a coin 128 times (or more, using von Neumann's approach) and encode your resulting number in to a seed phrase manually. The only third party code you need to use is a hash function to calculate the checksum. You obviously need to then use wallet software to turn that seed phrase in to a wallet, but even if you cannot read code yourself you can check two different pieces of software (such as use both Ian Coleman and Electrum) to check they both generate the same addresses from your seed phrase.

The whole point of generating a seed phrase is that seed phrases are easy to back up. Storing a deck of cards in a particular order is an incredibly risky idea. Anyone who finds it might use the deck without realizing what it is. You yourself might forget it is in order and absent-mindedly use it or shuffle it. Even if you are clumsy or slip when removing it from the packet and drop a few cards, or even if one end of the packet unexpectedly pops open, good luck trying to access your wallet again.

This is a mindset I have always disagreed with. People create all kinds of stupid methods for generating wallets. The most recent one I remember commenting on was using emojis, with people defending it by saying "Well, it's just for fun!" Even if the author created it "just for fun", there is a not insignificant chance that someone will use it and end up losing all their coins.