Why do used cars mpg drop vs new?

[stillsearching]

With exceptions for things I consider obvious like more friction, bad bearings, obvious malfunction...

Why should an older vehicle (say 15 years old and 150,000 miles) get lower mileage than a newer vehicle? Is it more in the engine or in the chassis? Should rebuilding the engine bring back the mileage or/and what other parts are most in need of freshening to get back to factory new numbers?

In specific i'm looking at getting and rebuilding a Geo Metro. I've heard for whatever reason they "never get the mileage used they did new", I don't know why this vehicle should be special or different than anything else. My 1999 Saturn SL gets the same identical mileage at 200k miles that it did almost new at 30k miles, zero change. Maybe I take better care of my cars than most. But I dont know why this should even matter all that much - short of mechanical damage causing increased drag I don't know why one should get less.