Most obviously for trust. TOFU is simple and while not fool-proof it's at least easy to think about the consequences. Using SSH with certificates (which somebody is bound to mention) is an afterthought and it shows.

There's also a very different default thinking about who is authenticating to who and why. In TLS the server must authenticate and clients largely do not. In SSH the server's "authentication" is often limited to just proving possession of some private key corresponding to a public key, but the client must provide a username and state up front how they're planning to authenticate before proceeding.

This is why that FIDO OpenSSH integration results in a file on your laptop (or whatever client) with local information whereas WebAuthn (FIDO integration for HTTPS) doesn't do anything like that.

As you'd expect although the underlying primitives aren't dissimilar (Diffie-Hellman style key agreement, AES encrypt everything, bind identity to encrypted session using public key signatures) the details are tailored to their application. TLS isn't a better SSH and SSH isn't a better TLS.