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I have studied gas engines a little. Even if the engine is maintaining its speed on the highway (low torque), it needs a minimal rpm so that an oil film remains between the crankshaft and the bearings. Going with an extra-low gear on the highway would be nice for fuel economy, but each time there is a small hill, the driver would need to downshift. Car manufacturers would probably be able to install a torque indicator requesting the driver to change gear (or an auto transmission would change all the time), but that would be annoying to most people, so we are stuck with approx. 2,800rpm at 60mph, which is somewhat ok for the average driver.
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