when we use technology like leased line to extend a lan over a wide area(like when connecting two offices such that hosts in both offices use private ip addresses) , why do we use encapsulations like ppp or hdlc...what can't we use the ethernet header to communicate because mac addresses are unique and can easily be used to identify hosts just like a small area lan...

this question may seem a bit absurd but it has been bugging me...so plz explain