Because the vast majority of their users don't want (or need, really) to bother with setting up GPG, making a key, adding their key to their account, etc.
Also, if users don't know the very basics of how Git works, they probably shouldn't be using it, and certainly not trusting it.
Nobody is talking about your "personal git" (I assume you mean your personal git repository). We are talking about two software programs: git, and GitHub. Both are ABSOLUTELY used by millions of people.
Also, if users don't know the very basics of how Git works, they probably shouldn't be using it, and certainly not trusting it.