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URL are a text representation of an IP address which is a more variable representation of a MAC address.

When you send a network packate to an IP, the router and other hardware is actually converting that too the MAC of the recieving network card. At the lowest level, the MAC is a required bit of information so even if you have encryption, that outer most layer of the onion is still going to present an unencrypted MAC (at least for the first recieving encryption device that is).

Encrypting the MAC would be like dropping a parcel in the post box with no address indicated; it's not going to get where it was meant to go.