That would require the transfer of files into a web server, which, given enough users, would take up a stuffload of space, ESPECIALLY if you're going to include addons in this.
It would also mean players want to have their stuff secure, so then you would have to pay a extra for a secure connection, and you'd also need some form of encryption scheme.
It would also be easy to inject a malicious file into the server if that was made, again especially if you're going to include addons.
And (correct me if I'm wrong) I don't think there's any consistent way of verifying the blid a key is used for, and verify that it is a valid and correct key, without actually looking at it, because players (and possibly players parents) would want the addon to not actually look at the key.