Changing behaviour
It's my guess - and it's only a guess - that the reason automakers strive for better mileage is because drivers/owners want it without having to change their behaviour, lifestyle or driving habits. It is possible to get greater mileage out of nearly any car by changing how one drives. But few there are who want to do that. So...sell more cars by developing what the buying public wants. Given that, what automaker truly intends to teach its customers how to get better mileage? They may give lip service to it, or even appear to be committed to it ("We at [automaker name here] constantly strive to improve the planet through better engineering, etc."), but it's not the number one reason they do it.
