Because quite simply dresses are not as practical to trousers! It simply fact.
Climbing, crawling, sliding down a wet slide, swinging on a bar, kicking a ball all are less comfortable nor offer protection of knees that trousers do. Most children s jeans have soft material with a soft waist. I don't see too many wearing proper jeans except for family events.
How many athletes who climb, run, cycle, wear skirts? If you are going for a hike what do you wear? I doubt its a skirt.
Children need to be absolutely encouraged to free play for as long as possible. And it is ridiculous how early people comment to girls about "minding their dresses" . Remember your daughter will hear these comments, she won't be isolated from them.