Honest Question

Why do snares become useless in 1v1's? All assassins can dash for the most part or jump. WHy doesnt the snare stop them the second it hits them but instead travels with them making snare useless? Even in teamfights, its not like i can stop someone from dyign if they can just jump to them in my snare. yes, do they have to hit their key first? yes, but those assassins dont have the smallest range. espcially considering the time it takes to stop, and snare, when youre tryign to run