It makes no sense. A lee sin activating his q the second time cannot be stopped by a root, nor can a dashing tryndamere, basically every dash cannot be stopped.
They get rooted only when they finish the dash and when they're ontop of you.
This mechanic allows to basically ignore root abilities and go straight through them to get ontop of enemy champions.
Once you're already ontop of the enemy you dashed to, you're in melee range and probably has already killed your target squishy champion who has a root ability.
So why is this a thing? Basically unrewarding to hit champion with a root during their dash because it doesn't affect them.
At certain angles, predicting those dashes can be extremely hard (e.g trying to save your teammate instead of yourself) and basically not rewarding at all.
Some thoughts i had about root effect when replying to comments:
When i imagine a root effect happening in an imaginary world i think of it like this:
>vines pop out of the ground entangling your feet and basically connecting you physically to the ground, denying all movement.
Clearly, the vines move with you in this imaginary world.. and that's why i think it should change.