The game is made this way because the spark of excitement from occasionally getting another + on your weapon triggers the brains reward system and makes you want to come back for more. That is how these games make money. Surprise chests, random amps, random drops... If the result were predictable, then people would lose interest and would have an easier time tracking where their money goes in the game.
"Psychologists call this PRE, or Partial Reinforcement Effect, in which the reward is only offered randomly, some of the time, such as in gambling. This leaves gamers hoping that just round the corner lies the suit of armour, gold, or some other reward they've been waiting for. That "reward" plays a part in the psychology of addiction is certain: Plenty of gamers will be familiar with the phrase "just ten more minutes".
http://theweek.com/articles/451660/psychology-video-game-addiction
А ты что надеялся за день снять 5-6 шмоток? Скажу тебе как истинный знаток топей. За неделю выполняя все легкие кв ты 100% снимешь две три вещи и какие они будут зависит от удачи, могут 15 лвл с надеваниен а может и красный сетовый шмот. Топи место для кача персонажа и зарабатывания голда. И идут туда не ради шмота а шмот идет бонусом.