It is because there are huge accumulation of impurities and malice hidden in different nooks and corners of their mind.
The so called impurities in the mind are unhealthy thoughts, habits or traits. Besides that illegitimate desire, anger, delusion, infatuation, negligence, carelessness, inadvertence, greed, jealousy, lust, malice, hatred, bias, envy, hypocrisy, negative emotions, mental agitation, arrogance, pride, prejudice, ignorance, lethargy, doubt, confusion, fear, worry, anxiety, frustration, grief, suspicion, dejection, conceit, temptations, stupor, hesitation, reluctance, forgetfulness, stupidity, superstition, oscillation, distraction, distortion, wrong hallucination, etc. are also impurities. These impurities cause disturbances in the mind by creating attachment and aversion and thus rob it of tranquility.
They are the urges, impulses and negative emotions.