You can judge distance from individual, over-bright dots? It's a well-known extremely difficult task for humans.
You have no size to compare, because the glow is larger than the headlight itself. You have large distances, so your parallax is minimal, and isn't very accurate anyway. It's dark, so on highways you frequently don't have any known references to compare against (like road width or car size). The closest you get is the distance between headlights and occlusion, which is unknown on any random vehicle and on more widely-spaced vehicles.
When you drive in the dark, if there is no ambient lighting, all you can generally see of a car approaching or behind is its lights, and you can definitely judge distance from that. This is noticeable particularly on motorways - it's very hard when you start to drive but becomes quite natural after a time.
You have no size to compare, because the glow is larger than the headlight itself. You have large distances, so your parallax is minimal, and isn't very accurate anyway. It's dark, so on highways you frequently don't have any known references to compare against (like road width or car size). The closest you get is the distance between headlights and occlusion, which is unknown on any random vehicle and on more widely-spaced vehicles.