Because if you meant keychain, all that does is make signing in more convenient because it recognizes your private key as a password and saves it for auto fill.

Using keychain to then auto fill your nsec into the app is the same as inputting the nsec yourself, just automated. Zero difference security wise.

As long as the app is open source and the nsec is only stored locally on your phone, that’s way better than directly putting your nsec in a web client, because browsers aren’t as secure and can be leaked.

Something like Passkey. Does it support secp256k1?