Because there is less light at night
Flick the courtesy light on, and it will be far easier to see in than out.
Window tints are not a one-way thing. They cut the same percentage of light in both directions at all times. Hence the limit of 15% tint on driving windows. They seem effective because there is less light inside a vehicle than out, partly due to the 100% blockage of light by panels, partly due to the blockage of light by any tints. Put an interior light on and they are easily seen through. Much like looking in a normal house window, you can't really see much, but as soon as the light is switched on you can see it all even where house windows generally have zero tint.
The difference will depend on the tint, Whilst the difference may not be immediately noticeable, especially as most fronts are already 15% tinted, try this simple test. Sit in the car, partly lower any window, look at the view, bet even the slight 15% tint on a front window is noticeable when compared directly in such a manner. do the same with a rear window that is more heavily tinted, and the difference is even more noticeable. The eyes are designed to adapt to available light, if there is less light available, they open wider to allow more light in, and of course the reverse.