And why is that?

Because franky1 is set in his old ways and doesn't trust BIP39 enough to convert properly his seed phrase into the private key (just like you don't trust LN closing properly a channel and settling your BTC balance on-chain).

Typical. Franky gonna franky!

I'm afraid this is a personality issue you need to resolve with a psychiatrist and has nothing to do with Lightning, Bitcoin or BIP39. Glad I helped!