Betraying my ignorance of public key cryptography

Here's something I don't understand.

Git uses ssh public/private key pairs for its wire-level network security.

Git uses gpg public/private key pairs for its commit-signing and tag-signing operations.

Why does git use two different keys for these two purposes?

Is that considered to be improved security?

Or is there a technical reason that git couldn't have simply used the same public/private key pair for both purposes?

And yes, I understand that in the wire-level network security case, it typically isn't git itself that is authenticating you using the SSH public key; it is actually the operating system that is doing that.

So I guess my question is: OK, but then why can't I use that same SSH key pair to sign my git commits and git tags? Why did git decide that I should use a different key pair for signing my git commits and tags?

Sorry for the naive question, but, as I admitted, I'm still rather ignorant about this cryptography stuff.

And, I see that other people are puzzled by this, too.