Because it doesn't solve anything without validation of email ownership. It doesn't matter if the commit says "Jen-Hsun Huang" or "ceo@nvidia.com". The commit usually contains both (in the "name " format) and both can be spoofed just as easily. The name associated with the email address of a Github account is actually harder to spoof because it requires creating an account with said email.
If people think "the" Linus Torvalds committed something just because they have a github account with the name Linus Torvalds, they're fools. Names aren't unique identifiers and they shouldn't be regarded as such.
There is something to be said got displaying the name and email address for signed commits on Github, to prevent creating an account and impersonating someone who actually signs their commits. In the default configuration, many version control systems will just add a "verified" badge to a commit if the email address of the signature matches an account with the same email address. That protection is obviously useless for unsigned commits, because there's no way to trust the author anyway, but for verified commits a clear indication of which email address created the commit makes sense. Alternatively, the github user account name could be used, of course, if such an account exists.
If you really want to find out the email address of the person who committed something, you can clone the repo and look at the git log manually, I suppose. I think hiding it was done to prevent shitty scrapers from collecting email addresses.