How do you keep distance to certain champs properly?

Darius, Sett, Morde. Maybe there is more.

They always get me despite me keeping the distance as far away as possible.

Darius especially always manages to grab me at the farthest pixel. Looking at the animation, his weapon did not even touch me visually, but somehow I got pulled.

Yet, when I watch teemo mains playing against them, you all manage to keep the distance perfectly to the point they cry.

How?