If I am an adc and you are a jungle taking safe lane cs
I'm going to farm your jungle camps. I'm going to take that farm because it is safer for me to do so, than trying to cs in a lane that is pushed. I can't simply walk anywhere I please and take cs. If i'm csing anywhere that is remotely isolated, I will get killed because at least 4/5 of the enemy team can 1v1 me. And everybody absolutely loves to gank the lone adc. You are the one who has the item that gives extra exp for farming the jungle. You are the one that is throwing away free exp. But we all need gold. And i'm going to take the farm that I can get.
EDIT: Something i'd like to add. I do this solely because it isn't very often that lane minions are safe to take alone. Mages, assassins, and tanks can usually farm most lanes alone without worrying about being ganked in a 1v1. At the very least, they can usually get away unless they got ganked by multiple people. But if an adc is trying to do that, they will usually not be able to get away. They don't have the defense to survive anything and the cc to outplay. They can't survive most 1v1s. Not to mention how much people love ganking lone adcs.

