Highway miles are much better on a car than city driving miles with stop and go driving.
At highway speeds the engine gets up to temperature better which helps burn off the moisture from the combustion process.
That keeps the oil in better shape and helps keep the engine lubricated better and at highway speeds the engine runs cooler due to air flow over the radiator and engine.
When driving around town the air doesn't get to the radiator or engine properly so the engine can run hotter as a result.
The hotter the oil the better the oil is at lubricating and prevents oil sludge buildup as well.
Highway driving is also good for an engine because the cylinders get hot enough to burn carbon deposits off and assuming you don't change speed too often the engine stays in lean burn mode so they don't form in the first place.
The transmission also has to shift less and so the less often the transmission has to shift into and out of speeds and gears the longer the transmission will last.