A moving target, going let's say 30 mph, you can't see the actual target, and if you miss, even if you hit the car, you've got a significant chance of hitting an innocent bystander. As a responsible gun owner, do you take that shot? Now let's also say they're returning fire, so now you've got to avoid the bullets, which chances are would distract you. Honestly, you think that anyone with less than military sniper training could take said shot responsibly?
In the normal course of events, snipers don't usually fire on moving targets. The range at which they operate, and the vision-limiting effects of telescoping sights, makes moving targets problematic.
Moving targets can be engaged with open sights, or with the new red-dot "holographic" sights.
If I'm standing on the side of the road and a car goes past me laterally at 30mph, I can't hit the driver at the moment the car passes me, the lateral movement is too great. After the car has passed, or before, relative lateral movement is reduced due to the angle. If i can see through the windshield and see the location of the driver, and I have a weapon capable of pentrating the windshield consistently, yes I can probably hit him. I'd prefer a semi-auto rifle for that task.
If he's shooting back, that adds a factor. I'm not going to shoot
while I am moving,
at a moving target. If I have cover and I am exposing only my head and hands to fire while aiming, I'll take the shot and try to put the incoming fire out of my mind.
If there are bystanders in the line of fire, either between me and the target, or beyond the target, I will not take the shot, because yes I
could miss. That would also apply in most cases whether the target was moving or not.
I have taken a shot on a live moving target, where there were houses beyond the target. The reason I took the shot is because I had a good angle and the round was going into the ground if I missed. I had about a half a second to make that call.