The slider is the portion of the zipper that pulls together the two chains to zip two sides together. On your jacket, the slider “belongs” to one side. It may mate with the other side, but it doesn’t belong with it. But look at your pants. The slider is not a citizen of either chain. The zipper, as a whole, is simply the zipper.
Well there is a big reason for the big difference.
Your jacket is not a pull over or a pull up, like pants are. If you have a pull over with a zipper, it is just like the pants zipper. Since niether side of pull overs, pull ups and pants is separate, the zipper slide is attached. Think of how awkward it would be if your pants zipper slider was not attached.
I can’t claim to have a coherent point there – but if there were one, you would have missed it.