I don’t understand why oncoming traffic and pedestrians all have a green light the same time as when you’re green for a left hand turn. Here in the Netherlands

I don’t understand why oncoming traffic and pedestrians all have a green light the same time as when you’re green for a left hand turn. Here in the Netherlands almost all traffic light intersections are set up so that when you’ve got green you’re good to go unless specifically indicated otherwise by signs. Not to mention roundabouts completely bypass this problem, though those might not have the traffic throughput needed in a city centre.