Simply put, the reason that the rear windows in most cars don’t roll down all the way is that the window storage compartment in the rear doors just isn’t big enough. Remember, when you roll down the windows, all that glass doesn’t just disappear into the ether, it has to go somewhere.
Car doors have a little cavity in them that holds the window glass when it’s rolled down. That cavity is bigger in the front doors because the doors themselves are typically larger. They’re big enough to accommodate both that cavity and all the other miscellaneous mechanics a car door needs.
The rear doors, however, are slightly smaller in size, usually not reaching all the way down to the bottom of the car’s body. Because the doors are physically smaller, there’s less room for a window cavity — there’s only enough spare space in a typical car door to push the window down about three-fourths of the way, while the rest of the space is needed for the passenger door’s mechanics.
It’s for this same reason that newer, larger cars have windows that actually can roll down all the way — the passenger doors are bigger. Bigger doors means more space for a window storage cavity, so the rear windows can safely roll all the way down without impacting the door’s functions.
Any safety improvements that arise from the window not rolling down all the way are simply a coincidence of design, or are potentially a happy accident from the limitations of passenger door manufacturing.