Well sometimes when you work for that one thing you want but you just never get it, you'll be quite frustrated. Maybe you really needed that one thing, say for example you really need a cross necklace from a mimic to complete a strategy you made, so you farm for hours but you never get it. By the time you've wasted away all your patience you'll be very fed up. When you never get it and it's your entire reason for playing, you lose your reason for playing and your interest. Meaning, because you farmed for hours you might take a break from terraria for maybe a couple or hours, days, weeks, months or longer. So in conclusion, RNG causes people to quit terraria.