The point is with the cigarette lighter you can choose an adapter that provides a sufficient amount of power. The cigarette outlet is rated for 5A at 12V which is plenty to amply charge the phone no matter what if you buy a decent adapter.
The current output of the USB port can vary from less than 1A to up to 2.4A at 5V which is not as much power. Some are so low that they're unable to charge the phone while its doing all those things. Since the iPhone 7 uses only one port for both power and audio/data out you're stuck with whatever the car manufacturer happened to provide in the USB port.
Let's face it, providing a robust USB port is not high on the priority list for car manufacturers. They probably have an intern throw that in as an afterthought and the primary purpose for it is transferring data, not power since every other device besides the iPhone has a separate port to provide power and doesn't usually rely on the USB.
