It's more like "why do I use Nostr when there are way more private communication channels?".

It's (Strike, Nostr, etc.) imperfect, but for the purpose it serves, it's currently the best option. And until a better option replaces it, I'll use it carefully for how it serves me best.

One day I hope to be using sovereign Lightning with other sovereign Lightning peers. Until then, I'll do that where I can and use the imperfect options where I can't.