It's all because of psychological "fairness".
A Jhin one-shotting half your health just because you walked in range for a split second, then not being able to retaliate because he zooms away with his incredible movement speed buff often times feels less fair than Twitch's power scenario where he will sneak up next to you, throw down his poison and slam 3-5 arrows in your chest then blow you up. Why? Because Twitch actually has to sit there to deal the damage, and you think to yourself "damn if I was more attentive I could have come out alive from that" or "if I just had a stun I would have been able to walk away!" The counterplay scenarios you can make in your head are a lot feasible than in Jhin's scenario, in which you would essentially just have to all-in him really hard and kill him before he can deal serious damage to you.